[TF Longformer] Add Multiple Choice, Seq Classification Model
# 🚀 Feature request

`modeling_longformer.py` has the classes `LongformerForSequenceClassification`, `LongformerForMultipleChoice` and `LongformerForTokenClassification` which are not present  in `modeling_tf_longformer.py` at the moment.
Those classes should be equally added to `modeling_tf_longformer.py`.
 
## Motivation

The pretrained weights for TFLongformer are available so that these classes could be used for finetuning.

## Your contribution

This issue is a good first issue because it is not too complicated to add these models. One should take a look at `modeling_tf_roberta.py` to see how these models are implemented for `TFRoberta` and implement them analogous for `TFLongformer`. Please make sure that the docstring is correct and that test are added for each class (again Roberta can serve as an example here, check out `test_modeling_tf_roberta.py`).

I am happy to guide interested community contributors through the PR and help them get it merged.

Hi !
I'd like to help and work on this if that's ok.<|||||>Awesome, feel free to open an issue :-) <|||||>Hello !

I'm a bit lost here. I've looked at `modeling_tf_roberta.py` and `modeling_longformer.py` to create the class `TFLongformerForSequenceClassification`. I'm not sure if I am going in the right direction here and same goes for the tests.
I used `python -m pytest -n auto --dist=loadfile -s -v ./tests/test_modeling_tf_roberta.py` to get an idea on what should I do for testing but it seems the test for `TFRobertaForSequenceClassification` is skipped and my test on the class I created (which is basically just a copy/paste of the roberta's test) is skipped too.

Here is a link to what I've done so far: https://github.com/Groskilled/transformers/commit/461ee6279433f94868332b1abbfe7875e19f243a

Am I on the right track ? And what am I missing on the tests ?

Sorry to ask such simple questions, it's my first time participating in an open source project.<|||||>No worries ;-). This looks alright! Could you open a PR so that we can see your changes directly on the PR? You can checkout this doc to understand how to do PRs: https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md. Would be great if you can ping me on the PR and then we look together!<|||||>HI @Groskilled and @patrickvonplaten, I have been playing a bit around this issue, as I have some familiarity with Keras/TF2 but no previous experience with transformers, and I was figuring out a way to start familiarising with them. As I am interested in classifying long documents Longformer is of interest to me.
I have a draft of my current changes [here](https://github.com/huggingface/transformers/compare/master...Zigur:tf-lonformer-good-first-release). The test suite seems to pass (using Python 3.7.5, they did not on Python 3.8.2 on my Mac machine), but I would need extensive feedback as I have mostly lifted code from `test_modeling_tf_roberta.py` and the testing counterpart.
If it is of interest, I can open a pull request with all the details, or @Groskilled you can feel free to cherry-pick part of it if it's useful for your own pull request (as you were working on this earlier on, apologies for the intromission)<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>Issue is still open! If stuck, feel free to take a look at the unfinished PR.