Added TF TransfoXL Sequence Classification
This PR implements Sequence classification for TF TransfoXL model.
TFTransfoXLForSequenceClassification uses the last token in order to do the classification, as other causal models (e.g. GPT-1 ,GPT-2) do.

Fixes #7623

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- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [x] Did you write any new necessary tests?

 @LysandreJik @jplu