[PERF] Add a util script to analyse perf results (#4107)
Summary:
This pull request introduces a helper script that takes results from
a test_train_mp_xxx.py test and do some statistical measurements. It
currently only prints Mean, Std Dev, Median, 90th% data for Rates with
warm up rounds results trimmed.
Test Plan:
python test/utils/train_mp_result_analyzer.py a.txt
