[WIP]  Create SequenceClassification MultipleChoice and TokenClassification for tf_longformer
Tests for the new classes are not passing and there is no test yet for TFLongformerClassificationHead (and I am not sure it's needed).
@patrickvonplaten 

Resolves #6401  
Hey @Groskilled - I added a couple of comments. Let's try to first make the torch tests pass. Let me know if you need more help after implementing the comments.<|||||>Closing PR due to inactivity