Report value for a step instead of epoch.
# What does this PR do?
Report an objective function value for a step instead of epoch to optuna.

## I made this modification for the following reason:
If "eval_steps" is less than steps per epoch, there maybe warnings: `optuna/trial/_trial.py:592: UserWarning: The reported value is ignored because this ‘step’ 0 is already reported.`. This is because the epoch granularity is too coarse. So "step" are more appropriate than "epoch" here.

## Who can review?
Anyone in the community is free to review the PR once the tests have passed. 
@sgugger @LysandreJik 

_The documentation is not available anymore as the PR was closed or merged._<|||||>> Looks good, thanks for fixing!
> 
> Can you just run `make style` on your branch to make sure the formatting check passes?

Done. @sgugger <|||||>Thanks a lot! (test failure is already fixed on main, so merging)