fix loading flax bf16 weights in pt (#14369)

* fix loading flax bf16 weights in pt

* fix clip test

* fix t5 test

* add logging statement

* Update src/transformers/modeling_flax_pytorch_utils.py

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* switch back to native any

* fix check for bf16 weights

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

src/transformers/modeling_flax_pytorch_utils.py

@@ -21,6 +21,7 @@ from typing import Dict, Tuple
 
 import numpy as np
 
+import jax
 import jax.numpy as jnp
 import transformers
 from flax.serialization import from_bytes
@@ -189,6 +190,19 @@ def load_flax_weights_in_pytorch_model(pt_model, flax_state):
         )
         raise
 
+    # check if we have bf16 weights
+    is_type_bf16 = flatten_dict(jax.tree_map(lambda x: x.dtype == jnp.bfloat16, flax_state)).values()
+    if any(is_type_bf16):
+        # convert all weights to fp32 if the are bf16 since torch.from_numpy can-not handle bf16
+        # and bf16 is not fully supported in PT yet.
+        logger.warning(
+            "Found ``bfloat16`` weights in Flax model. Casting all ``bfloat16`` weights to ``float32`` "
+            "before loading those in PyTorch model."
+        )
+        flax_state = jax.tree_map(
+            lambda params: params.astype(np.float32) if params.dtype == jnp.bfloat16 else params, flax_state
+        )
+
     flax_state_dict = flatten_dict(flax_state)
     pt_model_dict = pt_model.state_dict()
 

tests/test_modeling_flax_clip.py

@@ -227,6 +227,11 @@ class FlaxCLIPVisionModelTest(FlaxModelTesterMixin, unittest.TestCase):
     def test_save_load_to_base_pt(self):
         pass
 
+    # FlaxCLIPVisionModel does not have any base model
+    @is_pt_flax_cross_test
+    def test_save_load_bf16_to_base_pt(self):
+        pass
+
     @slow
     def test_model_from_pretrained(self):
         for model_class_name in self.all_model_classes:
@@ -332,6 +337,11 @@ class FlaxCLIPTextModelTest(FlaxModelTesterMixin, unittest.TestCase):
     def test_save_load_to_base_pt(self):
         pass
 
+    # FlaxCLIPVisionModel does not have any base model
+    @is_pt_flax_cross_test
+    def test_save_load_bf16_to_base_pt(self):
+        pass
+
     @slow
     def test_model_from_pretrained(self):
         for model_class_name in self.all_model_classes:

tests/test_modeling_flax_common.py

@@ -384,6 +384,35 @@ class FlaxModelTesterMixin:
                     max_diff = (base_params[key] - base_params_from_head[key]).sum().item()
                     self.assertLessEqual(max_diff, 1e-3, msg=f"{key} not identical")
 
+    @is_pt_flax_cross_test
+    def test_save_load_bf16_to_base_pt(self):
+        config, _ = self.model_tester.prepare_config_and_inputs_for_common()
+        base_class = FLAX_MODEL_MAPPING[config.__class__]
+
+        for model_class in self.all_model_classes:
+            if model_class == base_class:
+                continue
+
+            model = model_class(config)
+            model.params = model.to_bf16(model.params)
+            base_params_from_head = flatten_dict(unfreeze(model.params[model.base_model_prefix]))
+
+            # convert Flax model to PyTorch model
+            pt_model_class = getattr(transformers, model_class.__name__[4:])  # Skip the "Flax" at the beginning
+            pt_model = pt_model_class(config).eval()
+            pt_model = load_flax_weights_in_pytorch_model(pt_model, model.params)
+
+            # check that all base model weights are loaded correctly
+            with tempfile.TemporaryDirectory() as tmpdirname:
+                pt_model.save_pretrained(tmpdirname)
+                base_model = base_class.from_pretrained(tmpdirname, from_pt=True)
+
+                base_params = flatten_dict(unfreeze(base_model.params))
+
+                for key in base_params_from_head.keys():
+                    max_diff = (base_params[key] - base_params_from_head[key]).sum().item()
+                    self.assertLessEqual(max_diff, 1e-3, msg=f"{key} not identical")
+
     def test_jit_compilation(self):
         config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
 

tests/test_modeling_flax_t5.py

@@ -430,6 +430,36 @@ class FlaxT5ModelTest(FlaxModelTesterMixin, FlaxGenerationTesterMixin, unittest.
                     max_diff = (base_params[key] - base_params_from_head[key]).sum().item()
                     self.assertLessEqual(max_diff, 1e-3, msg=f"{key} not identical")
 
+    # overwrite since special base model prefix is used
+    @is_pt_flax_cross_test
+    def test_save_load_bf16_to_base_pt(self):
+        config, _ = self.model_tester.prepare_config_and_inputs_for_common()
+        base_class = FLAX_MODEL_MAPPING[config.__class__]
+
+        for model_class in self.all_model_classes:
+            if model_class == base_class:
+                continue
+
+            model = model_class(config)
+            model.params = model.to_bf16(model.params)
+            base_params_from_head = flatten_dict(unfreeze(model.params))
+
+            # convert Flax model to PyTorch model
+            pt_model_class = getattr(transformers, model_class.__name__[4:])  # Skip the "Flax" at the beginning
+            pt_model = pt_model_class(config).eval()
+            pt_model = load_flax_weights_in_pytorch_model(pt_model, model.params)
+
+            # check that all base model weights are loaded correctly
+            with tempfile.TemporaryDirectory() as tmpdirname:
+                pt_model.save_pretrained(tmpdirname)
+                base_model = base_class.from_pretrained(tmpdirname, from_pt=True)
+
+                base_params = flatten_dict(unfreeze(base_model.params))
+
+                for key in base_params_from_head.keys():
+                    max_diff = (base_params[key] - base_params_from_head[key]).sum().item()
+                    self.assertLessEqual(max_diff, 1e-3, msg=f"{key} not identical")
+
 
 @require_sentencepiece
 @require_tokenizers