Run jit legacy tests sequentially (#91518)
Fixes https://github.com/pytorch/pytorch/issues/91457. I have been re-running the 2 tests `test_jit_legacy` and `test_jit_fuser_legacy` in `jit_legacy` shard multiple times (100+) without any flaky issues found. I suspect that we might have a test parallelization flakiness here. So this PR runs these 2 tests serially. They takes less than 5 minutes to finish, so running them sequentially won't be an issue (https://hud.pytorch.org/hud/pytorch/pytorch/master/1?per_page=50&name_filter=jit_legacy)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91518
Approved by: https://github.com/clee2000
