Fix group offloading with block_level and use_stream=True (#11375)

* fix

* add tests

* add message check

src/diffusers/hooks/group_offloading.py

@@ -57,7 +57,7 @@ class ModuleGroup:
         non_blocking: bool = False,
         stream: Optional[torch.cuda.Stream] = None,
         record_stream: Optional[bool] = False,
-        low_cpu_mem_usage=False,
+        low_cpu_mem_usage: bool = False,
         onload_self: bool = True,
     ) -> None:
         self.modules = modules
@@ -498,6 +498,8 @@ def _apply_group_offloading_block_level(
             option only matters when using streamed CPU offloading (i.e. `use_stream=True`). This can be useful when
             the CPU memory is a bottleneck but may counteract the benefits of using streams.
     """
+    if stream is not None and num_blocks_per_group != 1:
+        raise ValueError(f"Using streams is only supported for num_blocks_per_group=1. Got {num_blocks_per_group=}.")
 
     # Create module groups for ModuleList and Sequential blocks
     modules_with_group_offloading = set()
@@ -521,7 +523,7 @@ def _apply_group_offloading_block_level(
                 stream=stream,
                 record_stream=record_stream,
                 low_cpu_mem_usage=low_cpu_mem_usage,
-                onload_self=stream is None,
+                onload_self=True,
             )
             matched_module_groups.append(group)
             for j in range(i, i + len(current_modules)):
@@ -529,12 +531,8 @@ def _apply_group_offloading_block_level(
 
     # Apply group offloading hooks to the module groups
     for i, group in enumerate(matched_module_groups):
-        next_group = (
-            matched_module_groups[i + 1] if i + 1 < len(matched_module_groups) and stream is not None else None
-        )
-
         for group_module in group.modules:
-            _apply_group_offloading_hook(group_module, group, next_group)
+            _apply_group_offloading_hook(group_module, group, None)
 
     # Parameters and Buffers of the top-level module need to be offloaded/onloaded separately
     # when the forward pass of this module is called. This is because the top-level module is not
@@ -560,8 +558,10 @@ def _apply_group_offloading_block_level(
         record_stream=False,
         onload_self=True,
     )
-    next_group = matched_module_groups[0] if len(matched_module_groups) > 0 else None
-    _apply_group_offloading_hook(module, unmatched_group, next_group)
+    if stream is None:
+        _apply_group_offloading_hook(module, unmatched_group, None)
+    else:
+        _apply_lazy_group_offloading_hook(module, unmatched_group, None)
 
 
 def _apply_group_offloading_leaf_level(

tests/hooks/test_group_offloading.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import contextlib
 import gc
 import unittest
 
@@ -20,6 +21,7 @@ import torch
 from diffusers.models import ModelMixin
 from diffusers.pipelines.pipeline_utils import DiffusionPipeline
 from diffusers.utils import get_logger
+from diffusers.utils.import_utils import compare_versions
 from diffusers.utils.testing_utils import require_torch_gpu, torch_device
 
 
@@ -58,6 +60,39 @@ class DummyModel(ModelMixin):
         return x
 
 
+# This model implementation contains one type of block (single_blocks) instantiated before another type of block (double_blocks).
+# The invocation order of these blocks, however, is first the double_blocks and then the single_blocks.
+# With group offloading implementation before https://github.com/huggingface/diffusers/pull/11375, such a modeling implementation
+# would result in a device mismatch error because of the assumptions made by the code. The failure case occurs when using:
+#   offload_type="block_level", num_blocks_per_group=2, use_stream=True
+# Post the linked PR, the implementation will work as expected.
+class DummyModelWithMultipleBlocks(ModelMixin):
+    def __init__(
+        self, in_features: int, hidden_features: int, out_features: int, num_layers: int, num_single_layers: int
+    ) -> None:
+        super().__init__()
+
+        self.linear_1 = torch.nn.Linear(in_features, hidden_features)
+        self.activation = torch.nn.ReLU()
+        self.single_blocks = torch.nn.ModuleList(
+            [DummyBlock(hidden_features, hidden_features, hidden_features) for _ in range(num_single_layers)]
+        )
+        self.double_blocks = torch.nn.ModuleList(
+            [DummyBlock(hidden_features, hidden_features, hidden_features) for _ in range(num_layers)]
+        )
+        self.linear_2 = torch.nn.Linear(hidden_features, out_features)
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        x = self.linear_1(x)
+        x = self.activation(x)
+        for block in self.double_blocks:
+            x = block(x)
+        for block in self.single_blocks:
+            x = block(x)
+        x = self.linear_2(x)
+        return x
+
+
 class DummyPipeline(DiffusionPipeline):
     model_cpu_offload_seq = "model"
 
@@ -212,3 +247,23 @@ class GroupOffloadTests(unittest.TestCase):
         pipe.enable_sequential_cpu_offload()
         with self.assertRaisesRegex(ValueError, "Cannot apply group offloading"):
             pipe.model.enable_group_offload(torch_device, offload_type="block_level", num_blocks_per_group=3)
+
+    def test_block_level_stream_with_invocation_order_different_from_initialization_order(self):
+        if torch.device(torch_device).type != "cuda":
+            return
+        model = DummyModelWithMultipleBlocks(
+            in_features=self.in_features,
+            hidden_features=self.hidden_features,
+            out_features=self.out_features,
+            num_layers=self.num_layers,
+            num_single_layers=self.num_layers + 1,
+        )
+        model.enable_group_offload(torch_device, offload_type="block_level", num_blocks_per_group=1, use_stream=True)
+
+        context = contextlib.nullcontext()
+        if compare_versions("diffusers", "<=", "0.33.0"):
+            # Will raise a device mismatch RuntimeError mentioning weights are on CPU but input is on device
+            context = self.assertRaisesRegex(RuntimeError, "Expected all tensors to be on the same device")
+
+        with context:
+            model(self.input)