Fix allreduce_coalesced tests in c10d (#25419)
Summary:
1. `test_allreduce_coalesced_stress` is flaky.
2. `test_allreduce_coalesced_checks` uses GPU but didn't claim so.
cc jfc4050
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25419
Differential Revision: D17119311
Pulled By: mrshenli
fbshipit-source-id: f560b126d6bc01363a14bdf6d697ecd55c4db468
