[WIP] Update: ignore padding support for TransfoXL training when n_clusters==0
Author: StefanHeng
Pull request: yes

# What does this PR do?

This PR solves [an issue](https://github.com/huggingface/transformers/issues/17446) I raised about TransformerXL. As @sgugger mentioned in [another issue](https://github.com/huggingface/transformers/issues/19914) I raised, he [says](https://github.com/huggingface/transformers/issues/19914#issuecomment-1293656206)

> I don't think TransformerXL supports FP16 as this is an old model with very specific code for the softmax layer. This won't be an issue we will fix ourselves given that Transformer-XL is not very used anymore, but if someone wants to make a PR, we'll review!

I'm using TransformerXL in a [research project](https://github.com/StefanHeng/Symbolic-Music-Generation) and disabling the adaptive softmax is an option I would like to explore.  So here I am. 

In the `n_clusters==0` branch, the current TransformerXL implementation does not work with padding (-100), it beaks at `.gather(1, labels)`. 

This PR solves that bug. 

I tested with my research data and confirmed my implementation is working. It's able to overfit the training data to up to 99% next-token prediction on multiple hyper-parameter setups, for #samples from 8 to 48, batch size from 48 to 64, epochs from 128 to 512. 

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Fixes # (issue)
[Issue 19914](https://github.com/huggingface/transformers/issues/19914)

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@sgugger 
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20326). All of your documentation changes will be reflected on that endpoint.

Thank you for your comments! 

> Thanks a lot for working on this! Can you make sure to run `make style` on your branch? You will also need to rebase on main to fix all the TensorFlow-related tests (they broke with last TensorFlow release).

Sure will do. 

>  Though it doesn't work the examples where we encourage labels=inputs["input_ids"]

@sgugger Can you clarify? Not sure what do you mean 

Please use -100 for padding in the labels as @patrickvonplaten indicated.

Sure, do we keep the 2 branches (line 113), i.e. support a case where we don't filter out -100? 

I'm fine either way. I added padding as an option because creating that extra mask token would consume extra memory. 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.

comment to reactivate thread