Add doctests for albert, bert, bigbird, mobilebert
# What does this PR do?

Add doctests for albert, bert, bigbird, mobilebert

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?

## Who can review?
@patrickvonplaten @ydshieh 