fix: make transformers import optional

The 'transformers' library is not listed in requirements.txt but was
imported at the top level in _utils.py, causing ImportError in environments
where it is not installed (e.g., CI).

This commit wraps the import in a try-except block to make it optional,
mirroring the logic in TokenizerWrapper.

Signed-off-by: majiayu000 <1835304752@qq.com>

Author: majiayu000

nano_graphrag/_utils.py

@@ -14,7 +14,10 @@
 import tiktoken
 
 
-from transformers import AutoTokenizer
+try:
+    from transformers import AutoTokenizer
+except ImportError:
+    AutoTokenizer = None
 
 logger = logging.getLogger("nano-graphrag")
 logging.getLogger("neo4j").setLevel(logging.ERROR)