[Feature] Support using FlashAttention2 on Ascend NPU (#36696)

* [Feature] Support using flash-attention on Ascend NPU

* Fix qwen3 and qwen3_moe moduler conversion mismatch

src/transformers/modeling_utils.py

@@ -2276,11 +2276,13 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin, PushToHubMix
             install_message = "Please refer to the documentation of https://huggingface.co/docs/transformers/perf_infer_gpu_one#flashattention-2 to install Flash Attention 2."
 
             if importlib.util.find_spec("flash_attn") is None:
+                # package `flash-attn` can not be installed on Ascend NPU, ignore related validation logic and early exit.
                 if is_torch_npu_available():
-                    recommend_message_npu = "You should use attn_implementation='sdpa' instead when using NPU. "
-                    raise ImportError(
-                        f"{preface} the package flash_attn is not supported on Ascend NPU. {recommend_message_npu}"
-                    )
+                    if not hard_check_only:
+                        config._attn_implementation = "flash_attention_2"
+
+                    logger.info("Detect using FlashAttention2 on Ascend NPU.")
+                    return config
                 else:
                     raise ImportError(f"{preface} the package flash_attn seems to be not installed. {install_message}")