T5 mlm Flax streaming example
# Added T5 mlm Flax streaming example

This PR adds an example script for T5 MLM Pretraining using 🤗 Datasets streaming feature. A new script `run_mlm_t5_flax_stream.py` is added in the `jax-projects/dataset-streaming` folder, and the `README.md` is updated accordingly with a training example for `t5-small` on the `mc4/en` corpus in streaming mode.

As mentioned in the Slack channel, I ran some preliminary tests on mc4/it and I get pretty weird results (train loss converges very early, eval metrics remain very low), possibly due to some problem with the adapted collating/tokenization, so this PR would greatly benefit from reviewing before merging.

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## Who can review?

@patrickvonplaten @patil-suraj

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