[deepspeed / testing] reset global state
This PR:
- adds a reset to the global state at the end of each in-pytest deepspeed test (and API to do that)
- fixes `test_load_best_model_zero2_fp16` to run the trainingargs first to get the ds config state right

cc: @ydshieh, who discovered the issues on CI

@sgugger 
_The documentation is not available anymore as the PR was closed or merged._