Stack:
    :black_circle:  #22037 Support sparse gradients in DistributedDataParallel  💛
    :white_circle:  #22036 Add sparse tensor allreduce  💛

This adds support for sparse gradients to the reducer as well as to
the DistributedDataParallel wrapper. Note that an out of band signal
is needed whether or not a dense parameter (e.g. an embedding) is
expected to receive a sparse gradient or not. This information is
passed to the bucket assignment computation routine and the reducer as
a vector of booleans. Every parameter for which we expect a sparse
gradient is assigned its own bucket, as we cannot easily group
multiple unrelated sparse tensors.

Differential Revision: D15926383