Fix group offloading synchronization bug for parameter-only GroupModule's (#12077)

* update

* update

* refactor

* fuck yeah

* make style

* Update src/diffusers/hooks/group_offloading.py
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

* Update src/diffusers/hooks/group_offloading.py

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

src/diffusers/hooks/group_offloading.py

@@ -245,7 +245,6 @@ class ModuleGroup:
                 param.data = self.cpu_param_dict[param]
             for buffer in self.buffers:
                 buffer.data = self.cpu_param_dict[buffer]
-
         else:
             for group_module in self.modules:
                 group_module.to(self.offload_device, non_blocking=False)
@@ -303,9 +302,23 @@ class GroupOffloadingHook(ModelHook):
         if self.group.onload_leader == module:
             if self.group.onload_self:
                 self.group.onload_()
-            if self.next_group is not None and not self.next_group.onload_self:
+
+            should_onload_next_group = self.next_group is not None and not self.next_group.onload_self
+            if should_onload_next_group:
                 self.next_group.onload_()
 
+            should_synchronize = (
+                not self.group.onload_self and self.group.stream is not None and not should_onload_next_group
+            )
+            if should_synchronize:
+                # If this group didn't onload itself, it means it was asynchronously onloaded by the
+                # previous group. We need to synchronize the side stream to ensure parameters
+                # are completely loaded to proceed with forward pass. Without this, uninitialized
+                # weights will be used in the computation, leading to incorrect results
+                # Also, we should only do this synchronization if we don't already do it from the sync call in
+                # self.next_group.onload_, hence the `not should_onload_next_group` check.
+                self.group.stream.synchronize()
+
         args = send_to_device(args, self.group.onload_device, non_blocking=self.group.non_blocking)
         kwargs = send_to_device(kwargs, self.group.onload_device, non_blocking=self.group.non_blocking)
         return args, kwargs