* NCCL based 1-bit Implementation + Refactor to add communication backends (#593)

* add nccl 1-bit optim.

* temporary commit to save stuff.

* Use dist collectives instead of mpi routines.

* remove old code for comm.

* Fix bugs. still does not work.

* modify to test the nccl side code path

* Initial gather impl. Works intra-node.

* Updates to comm. phase 2. nccl comm. passed the tests.

* refactor code to introduce nccl/mpi as backends for onebit adam.

* Refactor updates to test/engine.

* Fix compile/runtime errors.

* simplify support for nccl/mpi backends.

* Add missign file

* Add compression backend in constructor. Revert later.

* modify test with some perf counting.

* Implement a true non-blocking gather for nccl side.

* Revert "Add compression backend in constructor. Revert later."

This reverts commit df8c40d3.

* improve the 1-bit adam test.

* Refactor comm. and compression backend in 1-bit adam.

* Fix the test.

* Fix runtime errors and typos in nccl backend

* fix mpi backend. modify tests.

* modify nccl perf test.

* fix mpi side errors.

* Add an mpi perf test

* Sync DSE.

* Remove old collectives file.

* Undo a typo.

* Graceful failure for torch versions that don't support nccl pt2pt.