Alternate sharding (#119078)
Changes sharding to attempt to put all serial tests on as few shards as possible. Parallel tests are then distributed across all shards, with most of which likely ending up on the non serial shards
Example: 8 minutes of serial tests, 20 minutes of parallel tests, 2 proc per machine, 6 machines
-> 8 + 20/2 = 18 total minutes of tests
-> 18 / 6 machines = 3 min per machine
-> all serial tests should fit on 3 machines (3min, 3 min, 2min)
-> majority of parallel tests should go on last 4 machines, one of which is shared with the serial tests
Move serial tests to run first
If I want to move to a purely numbers based sharding, this ensures that parallel tests are run with parallel tests as much as possible instead of interleaving serial + parallel tests, which decreases effectiveness of parallelization, while also ensuring that test reordering is still mostly effective.
See https://hud.pytorch.org/pytorch/pytorch/commit/73e816ee809ad96fda3ee0013ee96273a44e84d4 for example logs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119078
Approved by: https://github.com/huydhn
