GitLab

* [docs]: add checkpoint_wrapper and many small fixes

* update copyright year

* rewrite using autograd and Variable execution queue to make the reduce automatic
* share buckets with OSS to remove duplication
* some speed still likely on the table since the speed vs. bucketing does not match expectations, could be a follow up

* adding a shard-aware GradScaler wrap, credits to Sean Naren for the idea
* adding stubs & explanations in the documentation

- removed experimental warning as we have validated it on cifar and
imagenet, transformer is looking good so far too.
- fixed API doc formatting
- make it consistent with the other code in the repo
- tested by making the doc locally and inspect the results

* various fixes, no more issues with `make html` and more API fields should be populated