[Model] Add FlexOlmo model implementation (#24923)

Signed-off-by: Shane A <shanea@allenai.org>

docs/models/supported_models.md

@@ -363,6 +363,7 @@ th {
 | `FalconForCausalLM` | Falcon | `tiiuae/falcon-7b`, `tiiuae/falcon-40b`, `tiiuae/falcon-rw-7b`, etc. | | ✅︎ | ✅︎ |
 | `FalconMambaForCausalLM` | FalconMamba | `tiiuae/falcon-mamba-7b`, `tiiuae/falcon-mamba-7b-instruct`, etc. | | ✅︎ | ✅︎ |
 | `FalconH1ForCausalLM` | Falcon-H1 | `tiiuae/Falcon-H1-34B-Base`, `tiiuae/Falcon-H1-34B-Instruct`, etc. | ✅︎ | ✅︎ | ✅︎ |
+| `FlexOlmoForCausalLM` | FlexOlmo | `allenai/FlexOlmo-7x7B-1T`, `allenai/FlexOlmo-7x7B-1T-RT`, etc. | | ✅︎ | ✅︎ |
 | `GemmaForCausalLM` | Gemma | `google/gemma-2b`, `google/gemma-1.1-2b-it`, etc. | ✅︎ | ✅︎ | ✅︎ |
 | `Gemma2ForCausalLM` | Gemma 2 | `google/gemma-2-9b`, `google/gemma-2-27b`, etc. | ✅︎ | ✅︎ | ✅︎ |
 | `Gemma3ForCausalLM` | Gemma 3 | `google/gemma-3-1b-it`, etc. | ✅︎ | ✅︎ | ✅︎ |

tests/models/registry.py

@@ -250,6 +250,7 @@ _TEXT_GENERATION_EXAMPLE_MODELS = {
     "Fairseq2LlamaForCausalLM": _HfExamplesInfo("mgleize/fairseq2-dummy-Llama-3.2-1B"),
     "FalconForCausalLM": _HfExamplesInfo("tiiuae/falcon-7b"),
     "FalconH1ForCausalLM": _HfExamplesInfo("tiiuae/Falcon-H1-0.5B-Base"),
+    "FlexOlmoForCausalLM": _HfExamplesInfo("allenai/Flex-reddit-2x7B-1T"),
     "GemmaForCausalLM": _HfExamplesInfo("google/gemma-1.1-2b-it"),
     "Gemma2ForCausalLM": _HfExamplesInfo("google/gemma-2-9b"),
     "Gemma3ForCausalLM": _HfExamplesInfo("google/gemma-3-1b-it"),

vllm/model_executor/models/flex_olmo.py

@@ -0,0 +1,157 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Inference-only FlexOlmo model compatible with HuggingFace weights."""
+
+from typing import Optional
+
+import torch
+from torch import nn
+
+from vllm.config import VllmConfig
+from vllm.distributed import get_tensor_model_parallel_world_size
+from vllm.logger import init_logger
+from vllm.model_executor.layers.fused_moe import FusedMoE
+from vllm.model_executor.layers.layernorm import RMSNorm
+from vllm.model_executor.layers.linear import ReplicatedLinear
+from vllm.model_executor.models.olmoe import OlmoeAttention, OlmoeForCausalLM
+from vllm.transformers_utils.configs import FlexOlmoConfig
+
+logger = init_logger(__name__)
+
+
+class FlexOlmoAttention(OlmoeAttention):
+    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
+        super().__init__(vllm_config=vllm_config, prefix=prefix)
+
+        hf_config = vllm_config.model_config.hf_config
+        assert isinstance(hf_config, FlexOlmoConfig)
+
+        self.k_norm = RMSNorm(
+            self.total_num_kv_heads * self.head_dim, eps=hf_config.rms_norm_eps
+        )
+        self.q_norm = RMSNorm(
+            self.total_num_heads * self.head_dim, eps=hf_config.rms_norm_eps
+        )
+
+
+class FlexOlmoMoE(nn.Module):
+    """A tensor-parallel MoE implementation for FlexOlmo that shards each expert
+    across all ranks.
+
+    Each expert's weights are sharded across all ranks and a fused MoE
+    kernel is used for the forward pass, and finally we reduce the outputs
+    across ranks.
+    """
+
+    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
+        super().__init__()
+
+        hf_config = vllm_config.model_config.hf_config
+        assert isinstance(hf_config, FlexOlmoConfig)
+
+        tp_size = get_tensor_model_parallel_world_size()
+
+        # Gate always runs at half / full precision for now.
+        self.gate = ReplicatedLinear(
+            hf_config.hidden_size,
+            hf_config.num_experts,
+            bias=False,
+            return_bias=False,
+            quant_config=None,
+            prefix=f"{prefix}.gate",
+        )
+
+        # Gate always runs at half / full precision for now.
+        self.experts = FusedMoE(
+            num_experts=hf_config.num_experts,
+            top_k=hf_config.num_experts_per_tok,
+            hidden_size=hf_config.hidden_size,
+            intermediate_size=hf_config.intermediate_size,
+            reduce_results=True,
+            renormalize=False,
+            quant_config=None,
+            tp_size=tp_size,
+            prefix=f"{prefix}.experts",
+        )
+
+        self.top_k = hf_config.num_experts_per_tok
+
+    def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
+        # NOTE: hidden_states can have either 1D or 2D shape.
+        orig_shape = hidden_states.shape
+        hidden_dim = hidden_states.shape[-1]
+        hidden_states = hidden_states.view(-1, hidden_dim)
+
+        # router_logits: (num_tokens, n_experts)
+        router_logits = self.gate(hidden_states)
+        # Warning: The experts mutate the hidden state input! This messes up
+        # basic things like the residual stream.
+        final_hidden_states = self.experts(
+            hidden_states=hidden_states.detach().clone(),
+            router_logits=router_logits.float(),
+        )
+
+        return final_hidden_states.view(orig_shape)
+
+
+class FlexOlmoDecoderLayer(nn.Module):
+    def __init__(self, *, vllm_config: VllmConfig, prefix: str = "") -> None:
+        super().__init__()
+        hf_config = vllm_config.model_config.hf_config
+        assert isinstance(hf_config, FlexOlmoConfig)
+
+        self.self_attn = FlexOlmoAttention(
+            vllm_config=vllm_config, prefix=f"{prefix}.self_attn"
+        )
+        self.post_attention_layernorm = RMSNorm(
+            hf_config.hidden_size, eps=hf_config.rms_norm_eps
+        )
+        self.post_feedforward_layernorm = RMSNorm(
+            hf_config.hidden_size, eps=hf_config.rms_norm_eps
+        )
+
+        self.mlp = FlexOlmoMoE(vllm_config=vllm_config, prefix=f"{prefix}.mlp")
+
+    def forward(
+        self,
+        positions: torch.Tensor,
+        hidden_states: torch.Tensor,
+        residual: Optional[torch.Tensor],
+    ) -> tuple[torch.Tensor, Optional[torch.Tensor]]:
+        # Attention block.
+        residual = hidden_states
+        hidden_states = self.self_attn(positions, hidden_states)
+        hidden_states = self.post_attention_layernorm(hidden_states)
+        hidden_states = hidden_states + residual
+
+        # MLP block.
+        residual = hidden_states
+        hidden_states = self.mlp(hidden_states)
+        hidden_states = self.post_feedforward_layernorm(hidden_states)
+        hidden_states = residual + hidden_states
+        return hidden_states, None
+
+
+class FlexOlmoForCausalLM(OlmoeForCausalLM):
+    fall_back_to_pt_during_load = False
+
+    def __init__(
+        self,
+        *,
+        vllm_config: VllmConfig,
+        prefix: str = "",
+        layer_type: type[nn.Module] = FlexOlmoDecoderLayer,
+    ):
+        super().__init__(vllm_config=vllm_config, prefix=prefix, layer_type=layer_type)

vllm/model_executor/models/olmoe.py

@@ -17,15 +17,14 @@
 from collections.abc import Iterable
 from functools import partial
 from itertools import islice
-from typing import Any, Optional, Union
+from typing import Optional, Union
 
 import torch
 from torch import nn
-from transformers import OlmoeConfig
 
 from vllm.attention import Attention
 from vllm.compilation.decorators import support_torch_compile
-from vllm.config import CacheConfig, VllmConfig
+from vllm.config import VllmConfig
 from vllm.distributed import (
     get_pp_group,
     get_tensor_model_parallel_rank,
@@ -117,20 +116,21 @@ class OlmoeMoE(nn.Module):
 
 
 class OlmoeAttention(nn.Module):
-    def __init__(
-        self,
-        hidden_size: int,
-        num_heads: int,
-        num_kv_heads: int,
-        rope_theta: float = 10000,
-        rope_scaling: Optional[dict[str, Any]] = None,
-        max_position_embeddings: int = 4096,
-        cache_config: Optional[CacheConfig] = None,
-        quant_config: Optional[QuantizationConfig] = None,
-        prefix: str = "",
-    ) -> None:
+    def __init__(self, *, vllm_config: VllmConfig, prefix: str = "") -> None:
         super().__init__()
-        self.hidden_size = hidden_size
+
+        config = vllm_config.model_config.hf_config
+        cache_config = vllm_config.cache_config
+        quant_config = vllm_config.quant_config
+
+        self.hidden_size = config.hidden_size
+        rope_theta = getattr(config, "rope_theta", 10000)
+        rope_scaling = getattr(config, "rope_scaling", None)
+        max_position_embeddings = getattr(config, "max_position_embeddings", 4096)
+
+        num_heads = config.num_attention_heads
+        num_kv_heads = config.num_key_value_heads
+
         tp_size = get_tensor_model_parallel_world_size()
         self.total_num_heads = num_heads
         assert self.total_num_heads % tp_size == 0
@@ -145,7 +145,7 @@ class OlmoeAttention(nn.Module):
             # the KV heads across multiple tensor parallel GPUs.
             assert tp_size % self.total_num_kv_heads == 0
         self.num_kv_heads = max(1, self.total_num_kv_heads // tp_size)
-        self.head_dim = hidden_size // self.total_num_heads
+        self.head_dim = self.hidden_size // self.total_num_heads
         self.q_size = self.num_heads * self.head_dim
         self.kv_size = self.num_kv_heads * self.head_dim
         self.scaling = self.head_dim**-0.5
@@ -153,7 +153,7 @@ class OlmoeAttention(nn.Module):
         self.max_position_embeddings = max_position_embeddings
 
         self.qkv_proj = QKVParallelLinear(
-            hidden_size,
+            self.hidden_size,
             self.head_dim,
             self.total_num_heads,
             self.total_num_kv_heads,
@@ -166,7 +166,7 @@ class OlmoeAttention(nn.Module):
         self.k_norm = RMSNorm(self.total_num_kv_heads * self.head_dim, eps=1e-5)
         self.o_proj = RowParallelLinear(
             self.total_num_heads * self.head_dim,
-            hidden_size,
+            self.hidden_size,
             bias=False,
             quant_config=quant_config,
         )
@@ -218,28 +218,15 @@ class OlmoeAttention(nn.Module):
 
 
 class OlmoeDecoderLayer(nn.Module):
-    def __init__(
-        self,
-        config: OlmoeConfig,
-        cache_config: Optional[CacheConfig] = None,
-        quant_config: Optional[QuantizationConfig] = None,
-        prefix: str = "",
-    ) -> None:
+    def __init__(self, *, vllm_config: VllmConfig, prefix: str = "") -> None:
         super().__init__()
+        config = vllm_config.model_config.hf_config
+        quant_config = vllm_config.quant_config
+
         self.hidden_size = config.hidden_size
-        rope_theta = getattr(config, "rope_theta", 10000)
-        rope_scaling = getattr(config, "rope_scaling", None)
-        max_position_embeddings = getattr(config, "max_position_embeddings", 4096)
 
         self.self_attn = OlmoeAttention(
-            hidden_size=self.hidden_size,
-            num_heads=config.num_attention_heads,
-            num_kv_heads=config.num_key_value_heads,
-            rope_theta=rope_theta,
-            rope_scaling=rope_scaling,
-            max_position_embeddings=max_position_embeddings,
-            cache_config=cache_config,
-            quant_config=quant_config,
+            vllm_config=vllm_config,
             prefix=f"{prefix}.self_attn",
         )
 
@@ -280,12 +267,16 @@ class OlmoeDecoderLayer(nn.Module):
 
 @support_torch_compile
 class OlmoeModel(nn.Module):
-    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
+    def __init__(
+        self,
+        *,
+        vllm_config: VllmConfig,
+        prefix: str = "",
+        layer_type: type[nn.Module] = OlmoeDecoderLayer,
+    ):
         super().__init__()
 
         config = vllm_config.model_config.hf_config
-        cache_config = vllm_config.cache_config
-        quant_config = vllm_config.quant_config
 
         self.vocab_size = config.vocab_size
         self.config = config
@@ -295,9 +286,7 @@ class OlmoeModel(nn.Module):
         )
         self.start_layer, self.end_layer, self.layers = make_layers(
             config.num_hidden_layers,
-            lambda prefix: OlmoeDecoderLayer(
-                config, cache_config, quant_config, prefix=prefix
-            ),
+            lambda prefix: layer_type(vllm_config=vllm_config, prefix=prefix),
             prefix=f"{prefix}.layers",
         )
         self.norm = RMSNorm(config.hidden_size, eps=1e-5)
@@ -339,7 +328,10 @@ class OlmoeModel(nn.Module):
                 {"hidden_states": hidden_states, "residual": residual}
             )
 
-        hidden_states, _ = self.norm(hidden_states, residual)
+        if residual is not None:
+            hidden_states, _ = self.norm(hidden_states, residual)
+        else:
+            hidden_states = self.norm(hidden_states)
         return hidden_states
 
     def get_expert_mapping(self) -> list[tuple[str, str, int, str]]:
@@ -455,14 +447,22 @@ class OlmoeForCausalLM(nn.Module, SupportsPP):
         ],
     }
 
-    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
+    def __init__(
+        self,
+        *,
+        vllm_config: VllmConfig,
+        prefix: str = "",
+        layer_type: type[nn.Module] = OlmoeDecoderLayer,
+    ):
         super().__init__()
         config = vllm_config.model_config.hf_config
         quant_config = vllm_config.quant_config
         self.config = config
         self.quant_config = quant_config
         self.model = OlmoeModel(
-            vllm_config=vllm_config, prefix=maybe_prefix(prefix, "model")
+            vllm_config=vllm_config,
+            prefix=maybe_prefix(prefix, "model"),
+            layer_type=layer_type,
         )
         self.lm_head = ParallelLMHead(
             config.vocab_size,

vllm/model_executor/models/registry.py

@@ -90,6 +90,7 @@ _TEXT_GENERATION_MODELS = {
     "Exaone4ForCausalLM": ("exaone4", "Exaone4ForCausalLM"),
     "FalconForCausalLM": ("falcon", "FalconForCausalLM"),
     "Fairseq2LlamaForCausalLM": ("fairseq2_llama", "Fairseq2LlamaForCausalLM"),
+    "FlexOlmoForCausalLM": ("flex_olmo", "FlexOlmoForCausalLM"),
     "GemmaForCausalLM": ("gemma", "GemmaForCausalLM"),
     "Gemma2ForCausalLM": ("gemma2", "Gemma2ForCausalLM"),
     "Gemma3ForCausalLM": ("gemma3", "Gemma3ForCausalLM"),

vllm/transformers_utils/config.py

@@ -74,6 +74,7 @@ _CONFIG_REGISTRY: dict[str, type[PretrainedConfig]] = LazyConfigDict(
     deepseek_vl_v2="DeepseekVLV2Config",
     deepseek_v3="DeepseekV3Config",
     deepseek_v32="DeepseekV3Config",
+    flex_olmo="FlexOlmoConfig",
     kimi_vl="KimiVLConfig",
     Llama_Nemotron_Nano_VL="Nemotron_Nano_VL_Config",
     RefinedWeb="RWConfig",  # For tiiuae/falcon-40b(-instruct)

vllm/transformers_utils/configs/__init__.py

@@ -17,6 +17,7 @@ from vllm.transformers_utils.configs.eagle import EAGLEConfig
 # tiiuae/falcon-7b(-instruct) models. Newer Falcon models will use the
 # `FalconConfig` class from the official HuggingFace transformers library.
 from vllm.transformers_utils.configs.falcon import RWConfig
+from vllm.transformers_utils.configs.flex_olmo import FlexOlmoConfig
 from vllm.transformers_utils.configs.jais import JAISConfig
 from vllm.transformers_utils.configs.kimi_vl import KimiVLConfig
 from vllm.transformers_utils.configs.lfm2_moe import Lfm2MoeConfig
@@ -45,6 +46,7 @@ __all__ = [
     "DeepseekV3Config",
     "DotsOCRConfig",
     "EAGLEConfig",
+    "FlexOlmoConfig",
     "RWConfig",
     "JAISConfig",
     "Lfm2MoeConfig",

vllm/transformers_utils/configs/flex_olmo.py

@@ -0,0 +1,77 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from transformers.configuration_utils import PretrainedConfig
+
+
+class FlexOlmoConfig(PretrainedConfig):
+    model_type = "flex_olmo"
+    keys_to_ignore_at_inference = ["past_key_values"]
+
+    def __init__(
+        self,
+        vocab_size=100352,
+        hidden_size=4096,
+        intermediate_size=11008,
+        num_hidden_layers=32,
+        num_attention_heads=32,
+        num_key_value_heads=None,
+        hidden_act="silu",
+        max_position_embeddings=4096,
+        initializer_range=0.02,
+        rms_norm_eps=1e-06,
+        use_cache=True,
+        pad_token_id=100277,
+        bos_token_id=None,
+        eos_token_id=100257,
+        tie_word_embeddings=False,
+        rope_theta=500000.0,
+        rope_scaling=None,
+        attention_bias=False,
+        attention_dropout=0.0,
+        num_experts_per_tok=5,
+        num_experts=7,
+        output_router_logits=False,
+        router_aux_loss_coef=0.01,
+        norm_topk_prob=False,
+        **kwargs,
+    ):
+        if "architectures" not in kwargs:
+            kwargs["architectures"] = ["FlexOlmoForCausalLM"]
+
+        super().__init__(
+            pad_token_id=pad_token_id,
+            bos_token_id=bos_token_id,
+            eos_token_id=eos_token_id,
+            tie_word_embeddings=tie_word_embeddings,
+            **kwargs,
+        )
+        self.vocab_size = vocab_size
+        self.max_position_embeddings = max_position_embeddings
+        self.hidden_size = hidden_size
+        self.intermediate_size = intermediate_size
+        self.num_hidden_layers = num_hidden_layers
+        self.num_attention_heads = num_attention_heads
+
+        # for backward compatibility
+        if num_key_value_heads is None:
+            num_key_value_heads = num_attention_heads
+
+        self.num_key_value_heads = num_key_value_heads
+        self.hidden_act = hidden_act
+        self.initializer_range = initializer_range
+        self.rms_norm_eps = rms_norm_eps
+        self.use_cache = use_cache
+        self.rope_theta = rope_theta
+        self.rope_scaling = rope_scaling
+        self.attention_bias = attention_bias
+        self.attention_dropout = attention_dropout
+        self.num_experts_per_tok = num_experts_per_tok
+        self.num_experts = num_experts
+        self.output_router_logits = output_router_logits
+        self.router_aux_loss_coef = router_aux_loss_coef
+        self.norm_topk_prob = norm_topk_prob
+        # Validate the correctness of rotary position embeddings parameters
+        # BC: if there is a 'type' field, move it to 'rope_type'.
+        if self.rope_scaling is not None and "type" in self.rope_scaling:
+            self.rope_scaling["rope_type"] = self.rope_scaling["type"]