[C/PyTorch] Removed MPI dependence in Userbuffers (#901)

* added DL framework callbacks for bootstrapping userbuffers without MPI
Signed-off-by: Alp Dener <adener@nvidia.com>

* removed userbuffers availability check in TE modules since userbuffers is now always compiled
Signed-off-by: Alp Dener <adener@nvidia.com>

* added comm+GEMM overlap example with LayerNormMLP
Signed-off-by: Alp Dener <adener@nvidia.com>

* lintin and review fixes
Signed-off-by: Alp Dener <adener@nvidia.com>

* linting and review fixes
Signed-off-by: Alp Dener <adener@nvidia.com>

* added header guards
Signed-off-by: Alp Dener <adener@nvidia.com>

* removed defunct userbuffers checks in build_utils and setup.py
Signed-off-by: Alp Dener <adener@nvidia.com>

* added exposed API in modules/base.py to __all__
Signed-off-by: Alp Dener <adener@nvidia.com>

* removed transformer_engine/CMakeLists.txt and shifted all TE/common compile into transformer_engine/common/CmakeLists.txt
Signed-off-by: Alp Dener <adener@nvidia.com>

---------
Signed-off-by: Alp Dener <adener@nvidia.com>
Co-authored-by: Tim Moon <4406448+timmoon10@users.noreply.github.com>

transformer_engine/pytorch/module/linear.py

@@ -729,9 +729,6 @@ class Linear(TransformerEngineBaseModule):
         self.ub_overlap_ag = ub_overlap_ag
         if ub_overlap_rs or ub_overlap_ag:
             assert ub_name is not None, "Userbuffer name [string] is not set."
-            assert (
-                tex.userbuf_comm_available()
-            ), "Userbuffer communication backend not available."
         self.ub_name = ub_name
         self.get_rng_state_tracker = get_rng_state_tracker
         self.rng_tracker_name = rng_tracker_name