Transformer-XL -100 label padding
Transformers version: 4.12.5
model: Transformer-XL / transfo_xl
url: https://github.com/huggingface/transformers/blob/master/src/transformers/models/transfo_xl/modeling_transfo_xl.py#L1011
issue:
In the docstring it is specified that: All labels set to ``-100`` are ignored (masked). 
However when passed to the model the error: index -100 is out of bounds for dimension 1 with size {vocab size} is thrown
I assume this is because of the different softmax function used by this model

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/master/CONTRIBUTING.md) are likely to be ignored.