Rename repo default branch to main
Rename repository default branch to `main` (instead of current `master`).

Once renamed, users will have to manually update their local repos:

- [ ] Upstream:
  ```
  git branch -m master main
  git fetch upstream main
  git branch -u upstream/main main
  git remote set-head upstream -a
  ```

- [ ] Origin:
Rename fork default branch as well at: https://github.com/USERNAME/lam/settings/branches
Then:
  ```
  git fetch origin main
  git remote set-head origin -a
  ```

CC: @sgugger