Returning outputs only when asked for for MaskFormer.
Author: Narsil
Pull request: yes

# What does this PR do?

Change the return output from `()` to `None` which seems more aligned with
the rest of the library.

Also `auxiliary_logits` seem optional and don't seem to be used by the feature
extractor, so this PR makes them optional too.

Note: I couldn't test the modeling tests, there seem to be no fast tests, and the slow tests are failing for reasons seemingly unrelated to this PR.

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Fixes # (issue)


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Comments:
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