Add sample softmax possibility to TransfoXL model for TransfoXL training
# 🚀 Feature request

TransfoXL samples the logits during training if required. At the moment TransfoXL can only be used whithout sampling from the logits during training. A partrly finished implementation can be found under the branch: `add_sampling_and_training_to_transfo_xl_models` .

## Motivation

To be able to train tranfoXL correctly.

## Your contribution

Already looked into the issue. Could try to implement it correctly with help from @thomwolf and @LysandreJik . Not a priority at the moment though.

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.
