[trainer] fix pt loss (#7748)

* fix pt loss

* robust

* fix

* test

src/llamafactory/model/model_utils/packing.py

@@ -43,11 +43,6 @@ import torch
 import torch.nn.functional as F
 
 from ...extras import logging
-from ...extras.packages import is_transformers_version_greater_than
-
-
-if is_transformers_version_greater_than("4.43.0"):
-    import transformers.modeling_flash_attention_utils
 
 
 if TYPE_CHECKING:
@@ -116,5 +111,7 @@ def configure_packing(model_args: "ModelArguments", is_trainable: bool) -> None:
     if not is_trainable or not model_args.block_diag_attn:
         return
 
+    import transformers.modeling_flash_attention_utils
+
     transformers.modeling_flash_attention_utils._get_unpad_data = get_unpad_data
     logger.info_rank0("Using block diagonal attention for sequence packing without cross-attention.")