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[PyTorch] Fixing hang in `initialize_ub()` for multi-node runs after PR901 removal of MPI-dependence (#986)

* Re-implementing PR901 (removing MPI-dependence in Userbuffers) with multi-node fixes

* passing data-parallel rank/size info from torch.distributed to userbuffers
Signed-off-by: Alp Dener <adener@nvidia.com>

* multi-node example working with UB_SKIPMC=1 but not with multicast
Signed-off-by: Alp Dener <adener@nvidia.com>

* fixed multi-node hang in initialize_ub(), updated comm+GEMM overlap example to support multi-node mixed tensor/data parallelism, added README
Signed-off-by: Alp Dener <adener@nvidia.com>

* fixed use case when Userbuffers is asked to allocate the TP overlap buffer with UB_SKIPMC=1
Signed-off-by: Alp Dener <adener@nvidia.com>

* corrected example problem to set device by local ordinal instead of global process rank
Signed-off-by: Alp Dener <adener@nvidia.com>

* double-free fix in userbuffers destructor
Signed-off-by: Alp Dener <adener@nvidia.com>

* removed unnecessary and incorrect torch.cuda.set_device(...)
Signed-off-by: Alp Dener <adener@nvidia.com>

* corrected inter-node ranks logic
Signed-off-by: Alp Dener <adener@nvidia.com>

* generalized node ID logic in initialize_ub to handle arbitrary world rank layouts within node
Signed-off-by: Alp Dener <adener@nvidia.com>

* added single-node comm+GEMM overlap unit tests
Signed-off-by: Alp Dener <adener@nvidia.com>

* LayerNormMLP example confirmed working with 2 nodes on Eos
Signed-off-by: Alp Dener <adener@nvidia.com>

* unit test cleanup
Signed-off-by: Alp Dener <adener@nvidia.com>

* corrected DP group ranks logic in LNMLP comm+GEMM overlap example
Signed-off-by: Alp Dener <adener@nvidia.com>

* corrected enums in unit test
Signed-off-by: Alp Dener <adener@nvidia.com>

* fixed incorrect Ubuf object init signature
Signed-off-by: Alp Dener <adener@nvidia.com>

* switched default backend for Userbuffer bootstrapping to Gloo with MPI and NCCL fallbacks, and initialize_ub option to manually select backend
Signed-off-by: Alp Dener <adener@nvidia.com>

* fixed all comm+GEMM overlap unit tests
Signed-off-by: Alp Dener <adener@nvidia.com>

* corrected all_gather use for Gloo backend
Signed-off-by: Alp Dener <adener@nvidia.com>

* changed userbuffers allgather callback to always use all_gather() instead of all_gather_into_tensor()
Signed-off-by: Alp Dener <adener@nvidia.com>

* restored and verified old MPI-based bootstrapping via NVTE_UB_WITH_MPI=1 option at compile time
Signed-off-by: Alp Dener <adener@nvidia.com>

* disabled scoped GIL release for comm+GEMM overlap algorithms
Signed-off-by: Alp Dener <adener@nvidia.com>

* avoid dist.init_device_mesh in comm+GEMM overlap example to support older PyTorch versions
Signed-off-by: Alp Dener <adener@nvidia.com>

* applied RS overlap FP8 fix from PR1004
Signed-off-by: Alp Dener <adener@nvidia.com>

* fixed segfault in Userbuffers destructor
Signed-off-by: Alp Dener <adener@nvidia.com>

* corrected comm+GEMM overlap unit test arguments
Signed-off-by: Alp Dener <adener@nvidia.com>

* fixed unit test run command for when Userbuffers is compiled with MPI
Signed-off-by: Alp Dener <adener@nvidia.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Refactored torch.distributed collectives into pure C++ callbacks
Signed-off-by: Alp Dener <adener@nvidia.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Signed-off-by: Alp Dener <adener@nvidia.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>