[Init] Make sure shape mismatches are caught early (#2847)

Improve init

src/diffusers/models/modeling_utils.py

@@ -579,10 +579,17 @@ class ModelMixin(torch.nn.Module):
                             " those weights or else make sure your checkpoint file is correct."
                         )
 
+                    empty_state_dict = model.state_dict()
                     for param_name, param in state_dict.items():
                         accepts_dtype = "dtype" in set(
                             inspect.signature(set_module_tensor_to_device).parameters.keys()
                         )
+
+                        if empty_state_dict[param_name].shape != param.shape:
+                            raise ValueError(
+                                f"Cannot load {pretrained_model_name_or_path} because {param_name} expected shape {empty_state_dict[param_name]}, but got {param.shape}. If you want to instead overwrite randomly initialized weights, please make sure to pass both `low_cpu_mem_usage=False` and `ignore_mismatched_sizes=True`. For more information, see also: https://github.com/huggingface/diffusers/issues/1619#issuecomment-1345604389 as an example."
+                            )
+
                         if accepts_dtype:
                             set_module_tensor_to_device(
                                 model, param_name, param_device, value=param, dtype=torch_dtype

tests/test_modeling_common.py

@@ -100,6 +100,30 @@ class ModelUtilsTest(unittest.TestCase):
 
         diffusers.utils.import_utils._safetensors_available = True
 
+    def test_weight_overwrite(self):
+        with tempfile.TemporaryDirectory() as tmpdirname, self.assertRaises(ValueError) as error_context:
+            UNet2DConditionModel.from_pretrained(
+                "hf-internal-testing/tiny-stable-diffusion-torch",
+                subfolder="unet",
+                cache_dir=tmpdirname,
+                in_channels=9,
+            )
+
+        # make sure that error message states what keys are missing
+        assert "Cannot load" in str(error_context.exception)
+
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            model = UNet2DConditionModel.from_pretrained(
+                "hf-internal-testing/tiny-stable-diffusion-torch",
+                subfolder="unet",
+                cache_dir=tmpdirname,
+                in_channels=9,
+                low_cpu_mem_usage=False,
+                ignore_mismatched_sizes=True,
+            )
+
+        assert model.config.in_channels == 9
+
 
 class ModelTesterMixin:
     def test_from_save_pretrained(self):