Restore _init_weights value in no_init_weights (huggingface#18504)

* Recover _init_weights value in no_init_weights

For potential nested use. 
In addition, users might modify private no_init_weights as well.

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Remove private variable change check

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/modeling_utils.py

@@ -106,12 +106,13 @@ def no_init_weights(_enable=True):
     TODO(Patrick): Delete safety argument `_enable=True` at next major version. .
     """
     global _init_weights
+    old_init_weights = _init_weights
     if _enable:
         _init_weights = False
     try:
         yield
     finally:
-        _init_weights = True
+        _init_weights = old_init_weights
 
 
 try: