Fix group-offloading bug (#13211)

* Implement synchronous onload for offloaded parameters

Add fallback synchronous onload for conditionally-executed modules.

* add test for new code path about group-offloading

* Update tests/hooks/test_group_offloading.py

Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

* use unittest.skipIf and update the comment

---------

Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/hooks/group_offloading.py

@@ -307,6 +307,17 @@ class GroupOffloadingHook(ModelHook):
         if self.group.onload_leader == module:
             if self.group.onload_self:
                 self.group.onload_()
+            else:
+                # onload_self=False means this group relies on prefetching from a previous group.
+                # However, for conditionally-executed modules (e.g. patch_short/patch_mid/patch_long in Helios),
+                # the prefetch chain may not cover them if they were absent during the first forward pass
+                # when the execution order was traced. In that case, their weights remain on offload_device,
+                # so we fall back to a synchronous onload here.
+                params = [p for m in self.group.modules for p in m.parameters()] + list(self.group.parameters)
+                if params and params[0].device == self.group.offload_device:
+                    self.group.onload_()
+                    if self.group.stream is not None:
+                        self.group.stream.synchronize()
 
             should_onload_next_group = self.next_group is not None and not self.next_group.onload_self
             if should_onload_next_group:

tests/hooks/test_group_offloading.py

@@ -566,3 +566,127 @@ class GroupOffloadTests(unittest.TestCase):
             "layers_per_block": 1,
         }
         return init_dict
+
+
+# Model with conditionally-executed modules, simulating Helios patch_short/patch_mid/patch_long behavior.
+# These modules are only called when optional inputs are provided, which means the lazy prefetch
+# execution order tracer may not see them on the first forward pass. This can cause a device mismatch
+# on subsequent calls when the modules ARE invoked but their weights were never onloaded.
+# See: https://github.com/huggingface/diffusers/pull/13211
+class DummyModelWithConditionalModules(ModelMixin):
+    def __init__(self, in_features: int, hidden_features: int, out_features: int, num_layers: int) -> None:
+        super().__init__()
+
+        self.linear_1 = torch.nn.Linear(in_features, hidden_features)
+        self.activation = torch.nn.ReLU()
+        self.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)
+
+        # These modules are only invoked when optional_input is not None.
+        # Output dimension matches hidden_features so they can be added after linear_1.
+        self.optional_proj_1 = torch.nn.Linear(in_features, hidden_features)
+        self.optional_proj_2 = torch.nn.Linear(in_features, hidden_features)
+
+    def forward(self, x: torch.Tensor, optional_input: torch.Tensor | None = None) -> torch.Tensor:
+        x = self.linear_1(x)
+        x = self.activation(x)
+        if optional_input is not None:
+            # Add optional projections after linear_1 so dimensions match (both hidden_features)
+            x = x + self.optional_proj_1(optional_input)
+            x = x + self.optional_proj_2(optional_input)
+        for block in self.blocks:
+            x = block(x)
+        x = self.linear_2(x)
+        return x
+
+
+class ConditionalModuleGroupOffloadTests(GroupOffloadTests):
+    """Tests for conditionally-executed modules under group offloading with streams.
+
+    Regression tests for the case where a module is not executed during the first forward pass
+    (when the lazy prefetch execution order is traced), but IS executed on subsequent passes.
+    Without the fix, the weights of such modules remain on CPU while the input is on GPU,
+    causing a RuntimeError about tensor device mismatch.
+    """
+
+    def get_model(self):
+        torch.manual_seed(0)
+        return DummyModelWithConditionalModules(
+            in_features=self.in_features,
+            hidden_features=self.hidden_features,
+            out_features=self.out_features,
+            num_layers=self.num_layers,
+        )
+
+    @parameterized.expand([("leaf_level",), ("block_level",)])
+    @unittest.skipIf(
+        torch.device(torch_device).type not in ["cuda", "xpu"],
+        "Test requires a CUDA or XPU device.",
+    )
+    def test_conditional_modules_with_stream(self, offload_type: str):
+        """Regression test: conditionally-executed modules must not cause device mismatch when using streams.
+
+        The model contains two optional Linear layers (optional_proj_1, optional_proj_2) that are only
+        executed when `optional_input` is provided. This simulates modules like patch_short/patch_mid/
+        patch_long in HeliosTransformer3DModel, which are only called when history latents are present.
+
+        When using streams, `LazyPrefetchGroupOffloadingHook` traces the execution order on the first
+        forward pass and sets up a prefetch chain so each module pre-loads the next one's weights.
+        Modules not executed during this tracing pass are excluded from the prefetch chain.
+
+        The bug: if a module was absent from the first (tracing) pass, its `onload_self` flag gets set
+        to False (meaning "someone else will onload me"). But since it's not in the prefetch chain,
+        nobody ever does — so its weights remain on CPU. When the module is eventually called in a
+        subsequent pass, the input is on GPU but the weights are on CPU, causing a RuntimeError.
+
+        We therefore must invoke the model multiple times:
+        1. First pass WITHOUT optional_input: triggers the lazy prefetch tracing. optional_proj_1/2
+            are absent, so they are excluded from the prefetch chain.
+        2. Second pass WITH optional_input: the regression case. Without the fix, this raises a
+            RuntimeError because optional_proj_1/2 weights are still on CPU.
+        3. Third pass WITHOUT optional_input: verifies the model remains stable after having seen
+            both code paths.
+        """
+
+        model = self.get_model()
+        model_ref = self.get_model()
+        model_ref.load_state_dict(model.state_dict(), strict=True)
+        model_ref.to(torch_device)
+
+        model.enable_group_offload(
+            torch_device,
+            offload_type=offload_type,
+            num_blocks_per_group=1,
+            use_stream=True,
+        )
+
+        x = torch.randn(4, self.in_features).to(torch_device)
+        optional_input = torch.randn(4, self.in_features).to(torch_device)
+
+        with torch.no_grad():
+            # First forward pass WITHOUT optional_input — this is when the lazy prefetch
+            # execution order is traced. optional_proj_1/2 are NOT in the traced order.
+            out_ref_no_opt = model_ref(x, optional_input=None)
+            out_no_opt = model(x, optional_input=None)
+            self.assertTrue(
+                torch.allclose(out_ref_no_opt, out_no_opt, atol=1e-5),
+                f"[{offload_type}] Outputs do not match on first pass (no optional_input).",
+            )
+
+            # Second forward pass WITH optional_input — optional_proj_1/2 ARE now called.
+            out_ref_with_opt = model_ref(x, optional_input=optional_input)
+            out_with_opt = model(x, optional_input=optional_input)
+            self.assertTrue(
+                torch.allclose(out_ref_with_opt, out_with_opt, atol=1e-5),
+                f"[{offload_type}] Outputs do not match on second pass (with optional_input).",
+            )
+
+            # Third pass again without optional_input — verify stable behavior.
+            out_ref_no_opt2 = model_ref(x, optional_input=None)
+            out_no_opt2 = model(x, optional_input=None)
+            self.assertTrue(
+                torch.allclose(out_ref_no_opt2, out_no_opt2, atol=1e-5),
+                f"[{offload_type}] Outputs do not match on third pass (back to no optional_input).",
+            )