Fix coding style and safety issues in CuBLAS nondeterministic unit test (#42627)
Summary:
Addresses some comments that were left unaddressed after PR https://github.com/pytorch/pytorch/issues/41377 was merged:
* Use `check_output` instead of `Popen` to run each subprocess sequentially
* Use f-strings rather than old python format string style
* Provide environment variables to subprocess through the `env` kwarg
* Check for correct error behavior inside the subprocess, and raise another error if incorrect. Then the main process fails the test if any error is raised
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42627
Reviewed By: malfet
Differential Revision: D22969231
Pulled By: ezyang
fbshipit-source-id: 38d5f3f0d641c1590a93541a5e14d90c2e20acec
