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

# What does this PR do?

I created a new PR since I couldn't seem to re-open the [original PR](https://github.com/huggingface/transformers/pull/20326). 

Requests in the original PR are handled. 

---------------------------------------------


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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Comments:
_The documentation is not available anymore as the PR was closed or merged._