Add a template for examples and apply it for mlm and plm examples
# 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 and a run_plm specific to XLNet. It runs with the same results as the old version.

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.