Add TFBartForSequenceClassification
Author: uglyboxer
Pull request: yes

# What does this PR do?
This adds a sequence classification head to the TensorFlow implementation of BART, following the pattern of `BartForSequenceClassification` (PyTorch version)

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Fixes # [Issue 19653](https://github.com/huggingface/transformers/issues/19653)

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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Library:

- benchmarks: @patrickvonplaten
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- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

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HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

_The documentation is not available anymore as the PR was closed or merged._

@ydshieh @sgugger Thank you both for your insightful reviews.  Pushed some changes and posed a question back.  

Thanks again @ydshieh

I'm afraid the special handling is necessary as the test `test_save_load_after_resize_token_embeddings`  does some extra magic to alter the input ids.  I took @sgugger 's suggestion to overwrite the test in the BartTester and move that logic into the test itself.    That should clear up common.

> Good to go with the nits, thanks for bearing with us!

Happy to take care of them!  This is my first PR, thanks for all the help, and the seamless process.

> > Good to go with the nits, thanks for bearing with us!
> 
> Happy to take care of them! This is my first PR, thanks for all the help, and the seamless process.

You are doing a great job! 💯