fix pytorch/xformers attention

This corrects a weird inconsistency with skip_reshape.
It also allows masks of various shapes to be passed, which will be
automtically expanded (in a memory-efficient way) to a size that is
compatible with xformers or pytorch sdpa respectively.

comfy/ldm/modules/attention.py

@@ -344,12 +344,9 @@ except:
     pass
 
 def attention_xformers(q, k, v, heads, mask=None, attn_precision=None, skip_reshape=False):
-    if skip_reshape:
-        b, _, _, dim_head = q.shape
-    else:
-        b, _, dim_head = q.shape
-        dim_head //= heads
-
+    b = q.shape[0]
+    dim_head = q.shape[-1]
+    # check to make sure xformers isn't broken
     disabled_xformers = False
 
     if BROKEN_XFORMERS:
@@ -364,41 +361,45 @@ def attention_xformers(q, k, v, heads, mask=None, attn_precision=None, skip_resh
         return attention_pytorch(q, k, v, heads, mask, skip_reshape=skip_reshape)
 
     if skip_reshape:
-         q, k, v = map(
-            lambda t: t.reshape(b * heads, -1, dim_head),
+        # b h k d -> b k h d
+        q, k, v = map(
+            lambda t: t.permute(0, 2, 1, 3),
             (q, k, v),
         )
+    # actually do the reshaping
     else:
+        dim_head //= heads
         q, k, v = map(
             lambda t: t.reshape(b, -1, heads, dim_head),
             (q, k, v),
         )
 
     if mask is not None:
+        # add a singleton batch dimension
+        if mask.ndim == 2:
+            mask = mask.unsqueeze(0)
+        if mask.ndim != 3:
+            raise ValueError(f"Bad mask shape {list(mask.shape)}. Valid shapes are [b, Nq, Nk], [1, Nq, Nk], or [Nq, Nk].")
+        # add a singleton heads dimension
+        mask = mask.unsqueeze(1)
+        # pad to a multiple of 8
         pad = 8 - mask.shape[-1] % 8
-        # if skip_reshape, then q, k, v have merged heads and batch size
-        if skip_reshape:
-            mask_out = torch.empty([q.shape[0], q.shape[1], mask.shape[-1] + pad], dtype=q.dtype, device=q.device)
-        # otherwise, we have separate heads and batch size
-        else:
-            mask_out = torch.empty([q.shape[0], q.shape[2], q.shape[1], mask.shape[-1] + pad], dtype=q.dtype, device=q.device)
+        # the xformers docs says that it's allowed to have a mask of shape (1, Nq, Nk)
+        # but when using separated heads, the shape has to be (B, H, Nq, Nk)
+        # in flux, this matrix ends up being over 1GB
+        # Therefore we create a mask with a singleton heads dimension, and then expand it (ie, create a view)
+        mask_out = torch.empty([mask.shape[0], 1, q.shape[1], mask.shape[-1] + pad], dtype=q.dtype, device=q.device)
 
         mask_out[..., :mask.shape[-1]] = mask
+        # doesn't this remove the padding again??
         mask = mask_out[..., :mask.shape[-1]]
+        mask = mask.expand(b, heads, -1, -1)
 
     out = xformers.ops.memory_efficient_attention(q, k, v, attn_bias=mask)
 
-    if skip_reshape:
-        out = (
-            out.unsqueeze(0)
-            .reshape(b, heads, -1, dim_head)
-            .permute(0, 2, 1, 3)
-            .reshape(b, -1, heads * dim_head)
-        )
-    else:
-        out = (
-            out.reshape(b, -1, heads * dim_head)
-        )
+    out = (
+        out.reshape(b, -1, heads * dim_head)
+    )
 
     return out
 
@@ -420,15 +421,31 @@ def attention_pytorch(q, k, v, heads, mask=None, attn_precision=None, skip_resha
             (q, k, v),
         )
 
-    if SDP_BATCH_LIMIT >= q.shape[0]:
+    if mask is not None:
+        # add a batch dimension if there isn't already one
+        if mask.ndim == 2:
+            mask = mask.unsqueeze(0)
+        # add a heads dimension if there isn't already one
+        if mask.ndim == 3:
+            mask = mask.unsqueeze(1)
+        mask = mask.expand(b, heads, -1, -1)
+
+
+    if SDP_BATCH_LIMIT >= b:
         out = torch.nn.functional.scaled_dot_product_attention(q, k, v, attn_mask=mask, dropout_p=0.0, is_causal=False)
         out = (
             out.transpose(1, 2).reshape(b, -1, heads * dim_head)
         )
     else:
-        out = torch.empty((q.shape[0], q.shape[2], heads * dim_head), dtype=q.dtype, layout=q.layout, device=q.device)
-        for i in range(0, q.shape[0], SDP_BATCH_LIMIT):
-            out[i : i + SDP_BATCH_LIMIT] = torch.nn.functional.scaled_dot_product_attention(q[i : i + SDP_BATCH_LIMIT], k[i : i + SDP_BATCH_LIMIT], v[i : i + SDP_BATCH_LIMIT], attn_mask=mask, dropout_p=0.0, is_causal=False).transpose(1, 2).reshape(-1, q.shape[2], heads * dim_head)
+        out = torch.empty((b, q.shape[2], heads * dim_head), dtype=q.dtype, layout=q.layout, device=q.device)
+        for i in range(0, b, SDP_BATCH_LIMIT):
+            out[i : i + SDP_BATCH_LIMIT] = torch.nn.functional.scaled_dot_product_attention(
+                q[i : i + SDP_BATCH_LIMIT],
+                k[i : i + SDP_BATCH_LIMIT],
+                v[i : i + SDP_BATCH_LIMIT],
+                attn_mask=None if mask is None else mask[i : i + SDP_BATCH_LIMIT],
+                dropout_p=0.0, is_causal=False
+            ).transpose(1, 2).reshape(-1, q.shape[2], heads * dim_head)
     return out