Fix issues in flax/nnx/rnglib.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: samanklesaria

flax/nnx/rnglib.py

@@ -474,13 +474,16 @@ def split(self, k: tp.Mapping[filterlib.Filter, int | tuple[int, ...]] | int | t
     for name, stream in self.items():
       for predicate, num_splits in split_predicates.items():
         if predicate((), stream):
-          if num_splits is None:
-            keys[name] = stream
-          else:
+          if num_splits is not None:
             keys[name] = stream.split(num_splits)
+          else:
+            # Fork to create a new stream without splitting the key, avoiding shared state.
+            keys[name] = stream.fork()
           break
-        else:
-          keys[name] = stream
+      else:
+        # If no predicate matches, fork the stream to avoid sharing state.
+        # This is consistent with the previous `fork` behavior.
+        keys[name] = stream.fork()
 
     return Rngs(**keys)