[torchelastic][1/n] Fix `caffe2.test.distributed.launcher.api_test` flaky tests (#68624)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68624
Fix `caffe2.test.distributed.launcher.api_test` flaky tests for opt-tsan mode.
The diff changes the default `mp.Process` invocation to use spawn context. `mp.Process` will uses `fork` method that is not compatible with `*san`.
Test Plan: CI
Reviewed By: d4l3k
Differential Revision: D32550578
fbshipit-source-id: f4767987e8e10a6a2ece3f86e48278f2dbaebe7c
