parameter `ignore_keys` of `trainer.predict` not accessible in `Trainer` or `TrainingArguments`
# 🚀 Feature request

The [`predict`](https://huggingface.co/transformers/main_classes/trainer.html#transformers.Trainer.predict) and [`evaluate`](https://huggingface.co/transformers/main_classes/trainer.html#transformers.Trainer.evaluate) methods of the Trainer class provide an excellent option of `ignore_keys`. Here is a small example:

```python
trainer.predict(dataset, ignore_keys=["ner_loss", "cls_loss", "ner_logits", "cls_logits"])
```

@sgugger 

This option is however, not accessible during the normal setup of defining `TrainingArguments` class nor the `Trainer` class so the a call to `trainer.train()` leads to errors during the mid-training evaluation.

## Motivation

I am unable to evaluate the model metrics on the validation set **during** the training to see if makes sense to continue.

## Your contribution

I am happy to make a PR if this is seen as a genuine problem. Like always, may be I am missing something.

We could add an argument for this (like `ignore_keys_for_eval`) yes. Let me know if you want to tackle this!<|||||>I very much wanna do this and will get right down to it!  👍 Admittedly though, I am relatively new to making contributions.<|||||>Hi! As a short update: I will not be able to work on this until 23rd of June... so if anyone wants to pick it up good otherwise I need 3 more weeks.<|||||>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.<|||||>OK so I'd like to work on this :)<|||||>Sure! Ping me when you open a PR :-)