Fix MaskFormer failing test on master
# What does this PR do?
This PR fixes a failing test on master, the fix was easy. In the test I was checking for `transformer_decoder_hidden_states` that only exists when `output_hidden_states == True`, what I needed to check was `transformer_decoder_last_hidden_state`. 

I have also made `output_hidden_states` depend on `config.use_auxiliary_loss` for the `MaskFormerModel` inside `MaskFormerForInstanceSegmentation` since we need all the `hidden_states` when computing the auxiliary loss
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