Fix sharding algo + test it (#53942)
Summary:
This PR:
1. moves sharding algorithm from run_test.py to framework_utils.py (let me know if you have a better place for it)
2. adds tests for the algorithm in test_testing.py
3. fixes the algorithm so that it doesn't tack on the unknown jobs all to the shard with the minimum time, but instead distributes them around the shards.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53942
Test Plan: python test/test_testing.py -k TestFrameworkUtils
Reviewed By: samestep
Differential Revision: D27047223
Pulled By: janeyx99
fbshipit-source-id: 824b20009c0bb707aa5361de445cdec795d5e3f1
