Raise error if loss can't be calculated - ViT MIM (#23872)

Raise error if loss can't be calculated

src/transformers/models/vit/modeling_vit.py

@@ -695,6 +695,13 @@ class ViTForMaskedImageModeling(ViTPreTrainedModel):
         ```"""
         return_dict = return_dict if return_dict is not None else self.config.use_return_dict
 
+        if bool_masked_pos is not None and (self.config.patch_size != self.config.encoder_stride):
+            raise ValueError(
+                "When `bool_masked_pos` is provided, `patch_size` must be equal to `encoder_stride` to ensure that "
+                "the reconstructed image has the same dimensions as the input."
+                f"Got `patch_size` = {self.config.patch_size} and `encoder_stride` = {self.config.encoder_stride}."
+            )
+
         outputs = self.vit(
             pixel_values,
             bool_masked_pos=bool_masked_pos,