PyTorch finalizes nccl2 backend with cleanup and AI-assisted code

PyTorch has merged a significant pull request (#188701) that applies inline review feedback to the new ProcessGroupNCCL (nccl2) distributed backend stack. Authored with an AI assistant (Claude), the PR cleans up multiple areas: it removes a vestigial `#if defined(IS_NCCLX)` guard, changes `getErrorString` to return `std::string_view` instead of `const char*`, collapses unused-parameter suppressions into a single `std::ignore = std::tie(...)`, and replaces raw `std::runtime_error` with `TORCH_CHECK`. These changes improve code safety, readability, and C++ compliance (avoiding pre-C++26 concatenation issues).

The naming feedback also resulted in renaming: the old TC suffix is gone, the backend lives in the `c10d::nccl2` namespace as `ProcessGroupNCCL` (exposed to Python as `ProcessGroupNCCL2` to avoid pybind name clashes), and the registrar registers a distinct 'nccl2' backend for CUDA that does not override the existing 'nccl' backend. Testing was performed with a targeted build and test run. This merge lays the groundwork for a cleaner, more maintainable NCCL communication stack for distributed PyTorch training, benefiting both developers and production users.