

TITLE
[Flax] Fix eval and data_args usage in streaming example

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# What does this PR do?

This PR fixes the evaluation loop in `run_mlm_flax_stream.py`. Current behavior didn't update the correct variable, which leads to data leakage during evaluation.

It also takes the opportunity to improve some `DataTrainingArguments` usages.

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It's a draft PR because there is an open improvement that could be made: the script splits train-eval based solely in `data_args.{dataset_name,num_eval_samples}`, but also accepts unused args `train_file, validation_file, train_ref_file, validation_ref_file, validation_split_percentage`. Other data args that are unused: `pad_to_max_length`, `line_by_line`.

My suggestion would be to remove all these unused args. May I proceed with that?


## Before submitting
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),  Request section?
- [x] Did you make sure to update the documentation with your changes?
    The script is mentioned in [`jax-projects/dataset-streaming/README`](https://github.com/huggingface/transformers/blob/95bab53868a91b4809bd5281a72b5f326853e31f/examples/research_projects/jax-projects/dataset-streaming/README.md#train-model), but no changes are required.


## Who can review?
@patrickvonplaten 
