update

src/diffusers/hooks/context_parallel.py

@@ -233,6 +233,21 @@ class ContextParallelGatherHook(ModelHook):
         return output[0] if is_tensor else tuple(output)
 
 
+class AllGatherFunction(torch.autograd.Function):
+    @staticmethod
+    def forward(ctx, tensor, dim, group):
+        ctx.dim = dim
+        ctx.group = group
+        ctx.world_size = torch.distributed.get_world_size(group)
+        ctx.rank = torch.distributed.get_rank(group)
+        return funcol.all_gather_tensor(tensor, dim, group=group)
+
+    @staticmethod
+    def backward(ctx, grad_output):
+        grad_chunks = torch.chunk(grad_output, ctx.world_size, dim=ctx.dim)
+        return grad_chunks[ctx.rank], None, None
+
+
 class EquipartitionSharder:
     @classmethod
     def shard(cls, tensor: torch.Tensor, dim: int, mesh: torch.distributed.device_mesh.DeviceMesh) -> torch.Tensor:
@@ -246,7 +261,7 @@ class EquipartitionSharder:
     @classmethod
     def unshard(cls, tensor: torch.Tensor, dim: int, mesh: torch.distributed.device_mesh.DeviceMesh) -> torch.Tensor:
         tensor = tensor.contiguous()
-        tensor = funcol.all_gather_tensor(tensor, dim, group=mesh.get_group())
+        tensor = AllGatherFunction.apply(tensor, dim, mesh.get_group())
         return tensor
 
 

src/diffusers/models/attention_dispatch.py

@@ -281,7 +281,8 @@ def dispatch_attention_fn(
         and not _AttentionBackendRegistry._is_context_parallel_enabled(backend_name)
     ):
         raise ValueError(
-            f"Backend {backend_name} does not support context parallelism, but a parallel configuration is provided."
+            f"Backend {backend_name} either does not support context parallelism or context parallelism "
+            f"was enabled with a world size of 1."
         )
 
     kwargs = {