remove allow-untyped-defs from torch/distributed/checkpoint/api.py (pytorch#144653)

Pull Request resolved: pytorch#144653
Approved by: https://github.com/Skylion007

Author: bobrenjc93

torch/distributed/checkpoint/api.py

@@ -1,4 +1,3 @@
-# mypy: allow-untyped-defs
 import traceback as tb
 from typing import Any, Dict
 
@@ -32,7 +31,7 @@ def failures(self) -> Dict[int, WRAPPED_EXCEPTION]:
         """Return a dictionary mapping node ranks to their associated exceptions in case of failure."""
         return self._failures
 
-    def __str__(self):
+    def __str__(self) -> str:
         str = f"CheckpointException ranks:{self._failures.keys()}\n"
         for rank, exc_pair in self._failures.items():
             exc, trace = exc_pair