Added TF CTRL Sequence Classification

[spatil6]

This PR implements Sequence classification for TF CTRL model.
TFCTRLForSequenceClassification uses the last token in order to do the classification, as other causal models (e.g. Transformer XL ,GPT-2) do.

Fixes #7623

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@jplu @LysandreJik

[LysandreJik]

Thanks for all your contributions @spatil6, they are very welcome!

[jplu]

LGTM! Thank you very much for your work!