[lint] improve ruff check (#11922)

Co-authored-by: Xiaoyu Zhang <35585791+BBuf@users.noreply.github.com>

python/sglang/srt/model_loader/weight_utils.py

@@ -110,6 +110,9 @@ def convert_bin_to_safetensor_file(
 
     dirname = os.path.dirname(sf_filename)
     os.makedirs(dirname, exist_ok=True)
+
+    from safetensors.torch import save_file
+
     save_file(loaded, sf_filename, metadata={"format": "pt"})
 
     # check file size