[Modular] Fix for custom block kwargs (#12561)

update

src/diffusers/modular_pipelines/modular_pipeline.py

@@ -335,7 +335,7 @@ class ModularPipelineBlocks(ConfigMixin, PushToHubMixin):
         )
         expected_kwargs, optional_kwargs = block_cls._get_signature_keys(block_cls)
         block_kwargs = {
-            name: kwargs.pop(name) for name in kwargs if name in expected_kwargs or name in optional_kwargs
+            name: kwargs.get(name) for name in kwargs if name in expected_kwargs or name in optional_kwargs
         }
 
         return block_cls(**block_kwargs)