C++ handler for gradient reduction (#18251)

C++ handler for gradient reduction (#18251)

Summary:
This commit adds the `c10d::Reducer` class that hooks into autograd
and performs gradient bucketing and reduction. These are the core
parts of `nn.parallel.DistributedDataParallel` that up to now were
only usable for CUDA models.

This should enable the following:

* Distributed data parallelism for models defined using the C++ frontend.
* Allow overlap of gradient computation and reduction for non-CUDA models.
* Enable distributed data parallelism for models with some unused parameters.

This does not include any logic for computing bucket assignment, which
can be done separately; either by observing autograd execution order
(this is what Apex does), or by assigning buckets based on some
maximum byte size, or both.

Also see #17757 and #13273.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18251

Reviewed By: mrshenli

Differential Revision: D14571899

Pulled By: pietern

fbshipit-source-id: 20f95eefd288dfe8cfffe0a28ca22fa7c9c3cd4c