make sure all torch._numpy tests run on CI (#108762)
- Add `if __name__ == "__main__": run_tests()` stanzas to test files in `torch_np` folder so that these tests run on CI
- Skip / xfail things smoked out by this change
- remove a stray python file which should not have been added to tests in the first place.
- fix einsum if opt_einsum is present
- add skips for older numpies
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108762
Approved by: https://github.com/lezcano
