Fix test_mps.py (#1256)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1256

A small fix to make the following test pass:

```
python3 -m unittest backends.apple.mps.test.test_mps --verbose -k mv3
```

imported-using-ghimport

Test Plan: Imported from OSS

Reviewed By: cccclai

Differential Revision: D51720241

Pulled By: larryliu0820

fbshipit-source-id: 6cdb2307608cf45269e5e7f97d42190b00ad48bc