remove attn output view kernel (#26680)

Signed-off-by: Boyuan Feng <boyuan@meta.com>
Signed-off-by: Boyuan Feng <fby.1994@gmail.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

vllm/attention/layer.py

@@ -346,7 +346,7 @@ class Attention(nn.Module, AttentionLayerBase):
 
         if self.use_output:
             output_shape = output_shape if output_shape is not None else query.shape
-            output = torch.zeros(output_shape, dtype=output_dtype, device=query.device)
+            output = torch.empty(output_shape, dtype=output_dtype, device=query.device)
             hidden_size = output_shape[-1]
             # Reshape the query, key, and value tensors.
             # NOTE(woosuk): We do this outside the custom op to minimize the
@@ -705,7 +705,7 @@ class MLAAttention(nn.Module, AttentionLayerBase):
                 self.calc_kv_scales(q, kv_c_normed, k_pe)
 
             if self.attn_backend.accept_output_buffer:
-                output = torch.zeros(output_shape, dtype=q.dtype, device=q.device)
+                output = torch.empty(output_shape, dtype=q.dtype, device=q.device)
                 self.impl.forward(
                     self,
                     q,
@@ -722,7 +722,7 @@ class MLAAttention(nn.Module, AttentionLayerBase):
                 )
         else:
             if self.attn_backend.accept_output_buffer:
-                output = torch.zeros(output_shape, dtype=q.dtype, device=q.device)
+                output = torch.empty(output_shape, dtype=q.dtype, device=q.device)
                 torch.ops.vllm.unified_mla_attention_with_output(
                     q,
                     kv_c_normed,

vllm/v1/attention/backends/flash_attn.py

@@ -530,7 +530,7 @@ class FlashAttentionImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         attn_type = self.attn_type
 

vllm/v1/attention/backends/flashinfer.py

@@ -857,7 +857,7 @@ class FlashInferImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         if self.bmm1_scale is None:
             self.bmm1_scale = layer._q_scale_float * layer._k_scale_float * self.scale

vllm/v1/attention/backends/flex_attention.py

@@ -767,7 +767,7 @@ class FlexAttentionImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
             # query = self.view_as_4d(query).permute(0, 2, 1, 3)
             # return torch.empty_like(query)
 

vllm/v1/attention/backends/rocm_aiter_fa.py

@@ -485,7 +485,7 @@ class AiterFlashAttentionImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         # IMPORTANT!
         # NOTE(woosuk): With piece-wise CUDA graphs, this method is executed in

vllm/v1/attention/backends/rocm_aiter_unified_attn.py

@@ -130,7 +130,7 @@ class RocmAiterUnifiedAttentionImpl(RocmAttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         assert attn_metadata.use_cascade is False
 

vllm/v1/attention/backends/rocm_attn.py

@@ -299,7 +299,7 @@ class RocmAttentionImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         assert attn_metadata.use_cascade is False
 

vllm/v1/attention/backends/tree_attn.py

@@ -379,7 +379,7 @@ class TreeAttentionImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         # Cache the input KVs.
         key_cache, value_cache = kv_cache.unbind(0)

vllm/v1/attention/backends/triton_attn.py

@@ -298,7 +298,7 @@ class TritonAttentionImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         assert attn_metadata.use_cascade is False
 

vllm/v1/attention/backends/xformers.py

@@ -354,7 +354,7 @@ class XFormersAttentionImpl(AttentionImpl):
 
         if attn_metadata is None:
             # Profiling run.
-            return output
+            return output.fill_(0)
 
         # Cache the input KVs.
         key_cache, value_cache = kv_cache.unbind(0)