Fix FSDP resume Initialization issue (#34032)

* Fix FSDP Initialization for resume training

* Added init_fsdp function to work with dummy values

* Fix FSDP initialization for resuming training

* Added CUDA decorator for tests

* Added torch_gpu decorator to FSDP tests

* Fixup for failing code quality tests

src/transformers/trainer.py

@@ -273,6 +273,39 @@ def _get_fsdp_ckpt_kwargs():
         return {}
 
 
+def _init_fsdp(model, accelerator, device):
+    """
+    Initialize Fully Sharded Data Parallel (FSDP) for the model.
+
+    This function is needed to properly initialize FSDP when resuming from a checkpoint.
+    It runs a forward pass with dummy inputs to ensure FSDP is fully initialized.
+    See https://github.com/huggingface/transformers/issues/31892 for more details.
+
+    Args:
+        model: The model to initialize with FSDP.
+        accelerator: The Accelerator object.
+        device: The device to run the model on.
+
+    Returns:
+        The initialized FSDP model.
+    """
+    model = accelerator.prepare(model)
+    model.train()
+    with torch.no_grad():
+        # Run a forward pass with dummy inputs to initialize FSDP
+        dummy_input = {
+            name: torch.ones(
+                (1, 512),
+                dtype=torch.long,
+                device=device,
+            )
+            for name in model.forward.__code__.co_varnames
+            if name != "self"
+        }
+        _ = model(**dummy_input)
+    return model
+
+
 if TYPE_CHECKING:
     import optuna
 
@@ -601,6 +634,10 @@ class Trainer:
                     " `Trainer`. Make sure the lines `import torch_xla.core.xla_model as xm` and"
                     " `model.to(xm.xla_device())` is performed before the optimizer creation in your script."
                 )
+
+        if self.is_fsdp_enabled:
+            self.model = _init_fsdp(self.model, self.accelerator, self.args.device)
+
         if (self.is_fsdp_xla_enabled or self.is_fsdp_enabled) and (
             self.optimizer is not None or self.lr_scheduler is not None
         ):

tests/trainer/test_trainer.py

@@ -4914,3 +4914,34 @@ class OptimizerAndModelInspectionTest(unittest.TestCase):
             param = next(model.parameters())
             group = trainer.get_optimizer_group(param)
             self.assertIn(param, group["params"])
+
+
+@require_torch_gpu
+@require_torch
+@require_accelerate
+class TestFSDPInitialization(unittest.TestCase):
+    def test_fsdp_initialization(self):
+        config = RegressionModelConfig(a=1, b=1, double_output=False)
+        model = RegressionPreTrainedModel(config)
+
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            training_args = TrainingArguments(
+                output_dir=tmp_dir,
+                fsdp=True,
+                fsdp_config={"min_num_params": 1},
+                no_cuda=True,
+            )
+            trainer = Trainer(model=model, args=training_args)
+
+            # Check for FSDP enabled
+            self.assertTrue(trainer.is_fsdp_enabled)
+
+            # Check if model is wrapped with FSDP
+            from torch.distributed.fsdp import FullyShardedDataParallel as FSDP
+
+            self.assertTrue(trainer.model, FSDP)
+
+            # Running a forward pass to ensure FSDP is initialized
+            dummy_input = torch.ones((1, 1), dtype=torch.float)
+            output = trainer.model(dummy_input)
+            self.assertTrue(output)