remove `noqa` comments in `gluonts/mx/__init__` by adding `__all__`

[Schmedu]

Issue #, if available: Mentioned in a comment in https://github.com/awslabs/gluon-ts/pull/1592 Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Please tag this pr with at least one of these labels to make our release process faster: BREAKING, new feature, bug fix, other change, dev setup

[Schmedu]

@jaheba am I missing something? Seems not to work with __all__ and without the comments

[jaheba]

Not sure why the docs workflow is complaining.