Make test case precision property (#40057)
Summary:
Make `common_utils.TestCase.precision` a property, because it is overriden as such in `common_device_type`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40057
Differential Revision: D22138385
Pulled By: malfet
fbshipit-source-id: 0e7c14654bf60f18f585efc61f96fdd0af23346f
