Change ValueError to NotImplementedError for unsupported partial saving replacement.

PiperOrigin-RevId: 807252930

Author: BlaziusMaximus

checkpoint/orbax/checkpoint/_src/handlers/base_pytree_checkpoint_handler.py

@@ -487,7 +487,7 @@ def _handle_diffs(keypath, diff):
         if diff.rhs is None:  # Leaf was not in the on-disk metadata
           additions.add(keypath)
         else:  # Leaf was also in the on-disk metadata
-          raise ValueError(
+          raise NotImplementedError(
               f'Key "{keypath}" was found in the on-disk PyTree metadata and'
               ' supplied item. Partial saving currently does not support'
               ' REPLACEMENT. Please reach out to the Orbax team if you need'