GitLab

* Fix gradient accumulation

Add ``is_scaled_loss`` flag to support both scaled / unscaled loss
Fix ``test_grad_accum`` and``test_set_num_gradients_to_accumulate``

* Add a method to scale grad for grad_accum using unscaled loss

- Revert the changes in `step` method
- Add a method `scale_grad_by_num_grads_to_accum`to handle gradient accumulation using unscaled loss more explicitly
- Add gradient tests

* Implement _compute_corr_mean_between_grads

* Improve tests and comments

* Use ubuntu-20.04 instead of latest

Use ubuntu-20.04 to fix the `arch x64 not found` issue
[Version 3.10 with arch x64 not found actions/setup-python#401](https://github.com/actions/setup-python/issues/401)

* Switch flake8 from gitlab to github

Flake8 was moved to Github
See discussions https://www.reddit.com/r/Python/comments/yvfww8/flake8_took_down_the_gitlab_repository_in_favor/

* Fix scikit-learn package

* Update PyTorch v...