Tracking variables other than loss during training
# 🚀 Feature request

Allow to track other variables during training with the [trainer](https://huggingface.co/transformers/main_classes/trainer.html).

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## Motivation
Often during training one wants to track variables other than just the loss. For example, the loss may be consisting of two different components and the user may want to track the two separately. As of now, the trainer can only track loss. It would be great if a user could simply pass the list of keys of auxiliary losses that they may want to track.   
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## Your contribution
I am happy to discuss and contribute code for this. 
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@sgugger 
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.