[fix] fix the issue when using fsdp2 with gradient checkpointing. (#9541)

Co-authored-by: jin-yongxu <jinyongxu@h-partners.com>

src/llamafactory/model/model_utils/checkpointing.py

@@ -18,6 +18,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import os
 import inspect
 from functools import WRAPPER_ASSIGNMENTS, partial, wraps
 from types import MethodType
@@ -152,6 +153,15 @@ def prepare_model_for_training(model: "PreTrainedModel", model_args: "ModelArgum
             if param.ndim == 1 and any(ln_name in name for ln_name in LAYERNORM_NAMES):
                 param.data = param.data.to(torch.float32)
 
+    if (
+        os.environ.get("ACCELERATE_USE_FSDP", "false").lower() == "true"
+        and int(os.environ.get("FSDP_VERSION", "1")) == 2
+        ):
+        model_args.use_reentrant_gc = False
+        logger.warning_rank0(
+            "You are using fsdp2, `use_reentrant_gc` has been set to False. "
+        )
+
     if not model_args.disable_gradient_checkpointing:
         if not getattr(model, "supports_gradient_checkpointing", False):
             logger.warning_rank0("Current model does not support gradient checkpointing.")