Adding performer fine-tuning research exampke
# What does this PR do?

This PR adds a performer fine-tuning research example based on `run_mlm_flax.py`. The user can fine-tune a Performer/FAVOR+ Bert starting from the Bert checkpoint or blank model of their choice, and compare it to a vanilla Bert model, also from a checkpoint or blank.

@patrickvonplaten
@sgugger - we want to do some fine-tuning experiments with the new performer model: https://arxiv.org/abs/2009.14794 before adding it to the `src/transformers/`. I think this is a good first place for it where we don't have to be super careful about the API choices yet. Is that fine for you?<|||||>Yes, this completely works for me!