parallelize at file granularity (#85770)
part two of https://github.com/pytorch/pytorch/pull/84961
tests files in parallel at the test file granularity
* 2 procs at a time
* number of tests ran changed by <200, possibly due to adding more tests on master between the base commit and head commit of the PR
* may cause flakiness, but I haven't seen it in my small sample size of this PR
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85770
Approved by: https://github.com/huydhn
