Fix attribute error for _gpus_arg_default loading checkpoint prior to 1.2.8 (#7043)

pytorch_lightning/utilities/argparse.py

@@ -286,6 +286,13 @@ def _gpus_allowed_type(x) -> Union[int, str]:
         return int(x)
 
 
+def _gpus_arg_default(x) -> Union[int, str]:  # pragma: no-cover
+    # unused, but here for backward compatibility with old checkpoints that need to be able to
+    # unpickle the function from the checkpoint, as it was not filtered out in versions < 1.2.8
+    # see: https://github.com/PyTorchLightning/pytorch-lightning/pull/6898
+    pass
+
+
 def _int_or_float_type(x) -> Union[int, float]:
     if '.' in str(x):
         return float(x)