Fix head masking for TFT5 models
* This PR fixes head masking in TFT5 models (#9859)
* This PR further fixes the name of an error message variable from `__HEAD_MASK_WARNING_MSG` to `_HEAD_MASK_WARNING_MSG` as the former one was not working properly and raised error (double underscore made troubles)

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Fixes: #9859

Reviewers: @jplu 
@stancld can you please rebase on master in order to solve the conflicts?<|||||>Thanks! The PR should be merged once @LysandreJik and @patrickvonplaten will have reviewed it.<|||||>Thanks for fixing it!