Migration guide from v3.x to v4.x (#8763)
* Migration guide from v3.x to v4.x
* Better wording
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* Sylvain's comments
* Better wording.
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
