Add coverage files to .gitignore (#61144)
Summary:
Fixes failures when coverage is turned on: https://github.com/pytorch/pytorch/runs/2966295169 https://github.com/pytorch/pytorch/runs/2964409741
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61144
Test Plan:
```bash
$ echo hi > test/.coverage.jit.1625168654.4504092
$ git status
$
```
Reviewed By: zhouzhuojie
Differential Revision: D29530709
Pulled By: driazati
fbshipit-source-id: 0e6a1cb217c4d48f14c0c58a546f98393d2b0392
