fix resuming from ckpt when using FSDP with FULL_STATE_DICT (#27891)

* fix resuming from ckpt when suing FSDP with FULL_STATE_DICT

* update tests

* fix tests

tests/fsdp/test_fsdp.py

@@ -41,6 +41,7 @@ from transformers.utils import is_accelerate_available, is_torch_bf16_available_
 
 if is_torch_available():
     from transformers.pytorch_utils import is_torch_greater_or_equal_than_2_1
+    from transformers.trainer import FSDP_MODEL_NAME
 else:
     is_torch_greater_or_equal_than_2_1 = False
 
@@ -211,6 +212,19 @@ class TrainerIntegrationFSDP(TestCasePlus, TrainerIntegrationCommon):
         # resume from ckpt
         checkpoint = os.path.join(output_dir, "checkpoint-115")
         resume_args = args + f"--resume_from_checkpoint {checkpoint}".split()
+
+        is_fsdp_ckpt = os.path.isdir(checkpoint) and (
+            # this checks the FSDP state dict when `SHARDED_STATE_DICT` is used
+            any(
+                FSDP_MODEL_NAME in folder_name
+                for folder_name in os.listdir(checkpoint)
+                if os.path.isdir(os.path.join(checkpoint, folder_name))
+            )
+            # this checks the FSDP state dict when `FULL_STATE_DICT` is used
+            or os.path.isfile(os.path.join(checkpoint, f"{FSDP_MODEL_NAME}.bin"))
+        )
+        self.assertTrue(is_fsdp_ckpt)
+
         logs_resume = self.run_cmd_and_get_logs(
             use_accelerate, sharding_strategy, launcher, script, resume_args, output_dir
         )