Remove redundant `eval()` & `train()` in `SleepStagerChambon2018`

[OverLordGoldDragon]

They don't affect shapes.

This doesn't seem to belong anywhere in whats_new, nor is it important, so I omitted, but feel free to edit.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.93%. Comparing base (a09a44a) to head (e085920).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #613 +/- ## ========================================== - Coverage 86.93% 86.93% -0.01% ========================================== Files 65 65 Lines 5796 5794 -2 ========================================== - Hits 5039 5037 -2 Misses 757 757 ```

[agramfort]

but is it a bad practice to move the model to eval mode when you are not training?

Message ID: @.*** com>

[agramfort]

I am not convinced we should merge this one.

[OverLordGoldDragon]

but is it a bad practice to move the model to eval mode when you are not training?

No, it's just confusing as it implies intent, i.e. that it'd be different if we removed the code. (I found it confusing and had to look into it).

Feel free to proceed as seen fit.