Tf longformer for sequence classification
Author: elk-cloner
Pull request: yes

# What does this PR do?

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implement SequenceClassification, MultipleChoice and TokenClassification classes for TFLongformer.
Resolves #6401 


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Comments:
@elk-cloner - thanks a lot for taking a look into this! 

Would be awesome to fix the TFLongformer related tests. There seem to be some obvious bug: `UnboundLocalError: local variable 'input_ids' referenced before assignment` . 

I'll do a longer review once these tests are fixed :-) Lemme know if you need help at some point.

@patrickvonplaten i have passed all the tests but got stuck in `test_inputs_embeds` when it's checking `TFLongformerForMultipleChoice` model, i debugged my code and found out that `inputs_embeds` shape is not same when `TFLongformerEmbeddings` get call from [here](https://github.com/elk-cloner/transformers/blob/28ab848279d31970c9f3390a480041eca2beee82/src/transformers/modeling_tf_longformer.py#L2232)(test_inputs_embeds) and [here](https://github.com/elk-cloner/transformers/blob/28ab848279d31970c9f3390a480041eca2beee82/tests/test_modeling_tf_common.py#L702)(TFLongformerForMultipleChoice), but don't know  how to fix it, can you help me ?

Hey @elk-cloner,

yeah this problem was not at all obvious! Thanks for letting me know :-) For Multiple Choice, we have to make sure that the position_ids stay 2-dimensional, which is only relevant for TFLongformer, but not for other TF models -> so we need this `if` fix here. 

Feel free to ping me again, when you're ready with the PR or need help :-) 

@patrickvonplaten all tests have passed, can you take a look ?

Hey @elk-cloner - the signature of the function calls should be done analogs to the one in other `modeling_tf_....py` files. Woud be great if you can fix that before we merge. 

Good to merge IMO! 

Checked the slow tests and everything passes. Great job @elk-cloner! Longformer is definitely not the easiest model

Would be awesome if @LysandreJik and @sgugger can take a final look, then we're good to merge.