Add init_weights method to FlaxMixin (#513)

* Add `init_weights` method to `FlaxMixin`

* Rn `random_state` -> `shape_state`

* `PRNGKey(0)` for `jax.eval_shape`

* No allow mismatched sizes

* Update src/diffusers/modeling_flax_utils.py

Co-authored-by: Suraj Patil <surajp815@gmail.com>

* Update src/diffusers/modeling_flax_utils.py

Co-authored-by: Suraj Patil <surajp815@gmail.com>

* docstring diffusers

Co-authored-by: Suraj Patil <surajp815@gmail.com>

src/diffusers/modeling_flax_utils.py

@@ -20,7 +20,7 @@ from typing import Any, Dict, Union
 import jax
 import jax.numpy as jnp
 import msgpack.exceptions
-from flax.core.frozen_dict import FrozenDict
+from flax.core.frozen_dict import FrozenDict, unfreeze
 from flax.serialization import from_bytes, to_bytes
 from flax.traverse_util import flatten_dict, unflatten_dict
 from huggingface_hub import hf_hub_download
@@ -183,6 +183,9 @@ class FlaxModelMixin:
         ```"""
         return self._cast_floating_to(params, jnp.float16, mask)
 
+    def init_weights(self, rng: jax.random.PRNGKey) -> Dict:
+        raise NotImplementedError(f"init_weights method has to be implemented for {self}")
+
     @classmethod
     def from_pretrained(
         cls,
@@ -227,10 +230,6 @@ class FlaxModelMixin:
             cache_dir (`Union[str, os.PathLike]`, *optional*):
                 Path to a directory in which a downloaded pretrained model configuration should be cached if the
                 standard cache should not be used.
-            ignore_mismatched_sizes (`bool`, *optional*, defaults to `False`):
-                Whether or not to raise an error if some of the weights from the checkpoint do not have the same size
-                as the weights of the model (if for instance, you are instantiating a model with 10 labels from a
-                checkpoint with 3 labels).
             force_download (`bool`, *optional*, defaults to `False`):
                 Whether or not to force the (re-)download of the model weights and configuration files, overriding the
                 cached versions if they exist.
@@ -394,6 +393,72 @@ class FlaxModelMixin:
         # flatten dicts
         state = flatten_dict(state)
 
+        params_shape_tree = jax.eval_shape(model.init_weights, rng=jax.random.PRNGKey(0))
+        required_params = set(flatten_dict(unfreeze(params_shape_tree)).keys())
+
+        shape_state = flatten_dict(unfreeze(params_shape_tree))
+
+        missing_keys = required_params - set(state.keys())
+        unexpected_keys = set(state.keys()) - required_params
+
+        if missing_keys:
+            logger.warning(
+                f"The checkpoint {pretrained_model_name_or_path} is missing required keys: {missing_keys}. "
+                "Make sure to call model.init_weights to initialize the missing weights."
+            )
+            cls._missing_keys = missing_keys
+
+        # Mistmatched keys contains tuples key/shape1/shape2 of weights in the checkpoint that have a shape not
+        # matching the weights in the model.
+        mismatched_keys = []
+        for key in state.keys():
+            if key in shape_state and state[key].shape != shape_state[key].shape:
+                raise ValueError(
+                    f"Trying to load the pretrained weight for {key} failed: checkpoint has shape "
+                    f"{state[key].shape} which is incompatible with the model shape {shape_state[key].shape}. "
+                )
+
+        # remove unexpected keys to not be saved again
+        for unexpected_key in unexpected_keys:
+            del state[unexpected_key]
+
+        if len(unexpected_keys) > 0:
+            logger.warning(
+                f"Some weights of the model checkpoint at {pretrained_model_name_or_path} were not used when"
+                f" initializing {model.__class__.__name__}: {unexpected_keys}\n- This IS expected if you are"
+                f" initializing {model.__class__.__name__} from the checkpoint of a model trained on another task or"
+                " with another architecture."
+            )
+        else:
+            logger.info(f"All model checkpoint weights were used when initializing {model.__class__.__name__}.\n")
+
+        if len(missing_keys) > 0:
+            logger.warning(
+                f"Some weights of {model.__class__.__name__} were not initialized from the model checkpoint at"
+                f" {pretrained_model_name_or_path} and are newly initialized: {missing_keys}\nYou should probably"
+                " TRAIN this model on a down-stream task to be able to use it for predictions and inference."
+            )
+        elif len(mismatched_keys) == 0:
+            logger.info(
+                f"All the weights of {model.__class__.__name__} were initialized from the model checkpoint at"
+                f" {pretrained_model_name_or_path}.\nIf your task is similar to the task the model of the checkpoint"
+                f" was trained on, you can already use {model.__class__.__name__} for predictions without further"
+                " training."
+            )
+        if len(mismatched_keys) > 0:
+            mismatched_warning = "\n".join(
+                [
+                    f"- {key}: found shape {shape1} in the checkpoint and {shape2} in the model instantiated"
+                    for key, shape1, shape2 in mismatched_keys
+                ]
+            )
+            logger.warning(
+                f"Some weights of {model.__class__.__name__} were not initialized from the model checkpoint at"
+                f" {pretrained_model_name_or_path} and are newly initialized because the shapes did not"
+                f" match:\n{mismatched_warning}\nYou should probably TRAIN this model on a down-stream task to be able"
+                " to use it for predictions and inference."
+            )
+
         # dictionary of key: dtypes for the model params
         param_dtypes = jax.tree_map(lambda x: x.dtype, state)
         # extract keys of parameters not in jnp.float32