New template for example and MLM example.
# What does this PR do?

This PR adds a cookiecutter template to add a new example and experiments with it to add the `run_mlm` new script. It runs with the same results as the old version. I'll also add a `run_plm` specific to XLNet then update the README and remove the old script.

Side note: the part for random masking applied in a data collator can become platform agnostic later on, if datasets adds a lazy map method.
