Force using fixed random seeds for flaky tests (#15515)
Some gradient-related tests fail frequently due to their math
properties. This PR fixes their random seed so that it's possible to
debug in the future.
Fixed
[AB#14605](https://aiinfra.visualstudio.com/6a833879-cd9b-44a4-a9de-adc2d818f13c/_workitems/edit/14605),
[AB#14604](https://aiinfra.visualstudio.com/6a833879-cd9b-44a4-a9de-adc2d818f13c/_workitems/edit/14604)
