Check for case where auxiliary_head is None in UperNetPreTrainedModel

[mmurray]

What does this PR do?

UperNetConfig has an option called use_auxiliary_head. When use_auxiliary_head is False, the auxiliary_head is set to None. However, UperNetPreTrainedModel assumes that auxiliary_head is not None, resulting in an error when loading a pretrained upernet model with use_auxiliary_head=False (see issue #25513 ).

This pull request updates UperNetPreTrainedModel to appropriately handle the case where auxiliary_head is None.

Fixes #25513

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@amyeroberts

[sgugger]

Thanks for the fixes!