reorganize sparse_nn_partition (#47283)
Summary:
This PR moves combine_partitions_based_on_size and find_partition_to_combine_based_on_size to sparse_nn_partition since they are both only used by sparse_nn_partition
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47283
Reviewed By: gcatron
Differential Revision: D24707864
Pulled By: scottxu0730
fbshipit-source-id: 183fe945e477e16301d7f489103287eb9d8a30af
