Use smaller N to speed up TestForeach (#45785)
Summary:
Between September 25 and September 27, approximately half an hour was added to the running time of `pytorch_linux_xenial_cuda10_2_cudnn7_py3_gcc7_test`. Judging from the CircleCI data, it looks like the majority of the new time was added by the following PRs:
- https://github.com/pytorch/pytorch/issues/44550
- https://github.com/pytorch/pytorch/issues/45298
I'm not sure what to do about https://github.com/pytorch/pytorch/issues/44550, but it looks like https://github.com/pytorch/pytorch/issues/45298 increased the `N` for `TestForeach` from just 20 to include both 30 and 300. This PR would remove the 300, decreasing the test time by a couple orders of magnitude (at least when running it on my devserver), from over ten minutes to just a few seconds.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45785
Reviewed By: malfet
Differential Revision: D24094782
Pulled By: samestep
fbshipit-source-id: 2476cee9d513b2b07bc384de751e08d0e5d8b5e7
