add user vs internal msg support in common_utils.TestCase (#48935)
Summary:
should fixes https://github.com/pytorch/pytorch/issues/48879.
To test the effect of the messages: make test break, such as add `self.assertEqual(1, 2, "user_msg")` to any test
* Before:
```
AssertionError: False is not true : user_msg
```
* After
```
AssertionError: False is not true : Scalars failed to compare as equal! Comparing 1 and 2 gives a difference of 1, but the allowed difference with rtol=0 and atol=0 is only 0!
user_msg;
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48935
Reviewed By: samestep
Differential Revision: D25382153
Pulled By: walterddr
fbshipit-source-id: 95633a9f664f4b05a28801786b12a10bd21ff431
