Refactor checkpoint name in BERT and MobileBERT
# What does this PR do?

Linked to #10193, this PR gives an example on how to refactor the checkpoint names in one private constant and use the `# Copied from` syntax to make the task-specific models that are copies of each other properly watched by our tooling.

It adds the option in check-copies to:
- put multiple statements behind the with: so for instance `with Bert->MobileBert, bert->mobilebert, BERT->MOBILEBERT`
- have the option to do all possible casings (like in the example above) by just adding `all-casing`:  `with Bert->MobileBert all-casing`.

It also fixes an existing bug when the first line of the function/class copied from was empty.

