Deprecate `TransfoXL` (#27607)
* fix
* fix
* trigger
* Apply suggestions from code review
Co-authored-by: Lysandre Debut <hi@lysand.re>
* tic
* revert
* revert
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Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>
Co-authored-by: Lysandre Debut <hi@lysand.re>
