feat: support NCCL on Iluvatar

[Ziminli]

Summary

Add NCCL backend support for Iluvatar by reusing the shared NCCL CCL abstraction introduced in the current codebase. This PR adds the Iluvatar NCCL provider alias, marks BackendType::kNccl with Device::Type::kIluvatar as a supported backend/device combination, and updates CMake so NCCL can be discovered and required for either NVIDIA or Iluvatar builds.

This keeps the platform/backend separation intact: Iluvatar does not duplicate NCCL operation implementations, and it can use the existing src/backends/ccl/nccl implementation through a small provider-specific api.h.

Changes

• Iluvatar NCCL Provider

  • Add src/backends/ccl/nccl/iluvatar/api.h.
  • Bind CclApi<BackendType::kNccl, Device::Type::kIluvatar> to NcclApi<Device::Type::kIluvatar>.

• Backend/device support

  • Update src/backend_device_map.h so BackendType::kNccl is accepted with Device::Type::kIluvatar.

• CMake detection and linking

  • Allow NCCL auto-detection when either WITH_NVIDIA or WITH_ILUVATAR is enabled.
  • Add CoreX-style NCCL search hints under /usr/local/corex for Iluvatar builds.
  • Update the explicit WITH_NCCL requirement so NCCL accepts NVIDIA or Iluvatar GPU support.
  • Link examples against NCCL_LIB when WITH_NCCL is enabled, so NCCL symbols are available to example targets.

Platform and Backend Affected

Platform

• CPU
• NVIDIA GPU
• Iluvatar GPU
• MetaX GPU
• Moore Threads GPU
• Cambricon MLU

Backend

• OpenMPI
• MPICH
• NCCL

Performance Impact

• No performance impact
• Performance improved
• Performance regression possible

N/A..

Known Issues & Future Work

• This PR assumes Iluvatar's NCCL-compatible library exposes the NCCL APIs used by the shared NcclApi implementation. If Iluvatar NCCL later diverges from NVIDIA NCCL for specific APIs or semantics, follow-up work can override only the divergent pieces under src/backends/ccl/nccl/iluvatar.

Test Results

Test Involved Platform

• CPU
• NVIDIA GPU
• Iluvatar GPU
• MetaX GPU
• Moore Threads GPU
• Cambricon MLU

Test Involved Backend

• OpenMPI
• MPICH
• NCCL

Iluvatar with NCCL:
ccl_all_reduce.log

Iluvatar with NCCL+OpenMPI:
ccl_mpi_hybrid_all_reduce.log

NVIDIA with NCCL:
ccl_all_reduce.log

NVIDIA with NCCL+OpenMPI:
ccl_mpi_hybrid_all_reduce.log

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Checklist

Every contributor must verify every item below before requesting
review. Tick each box only after the check has actually been performed —
do not tick speculatively. If an item truly does not apply, replace the
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Title, Branch, and Commits

• PR title follows Conventional Commits (e.g. feat: …, fix(nccl): …).
• Branch name follows <type>/xxx-yyyy-zzzz where <type> matches the PR title's Conventional Commits type and words are joined with hyphens (see CONTRIBUTING.md §Branches).
• Each commit message follows Conventional Commits.
• Small PR is a single squashable commit; or, for a large PR, every commit is meaningful, well-formed, and independently reviewable (see CONTRIBUTING.md §Pull Requests).
• No stray merge commits from master — the branch is rebased cleanly on top of the current master.
• No fixup! / squash! / wip commits remain.

Scope and Design

• Changes are minimal — no unrelated modifications were introduced (CONTRIBUTING.md §Code/General).
• No dead code, commented-out blocks, debug prints, printf/std::cout/print(...) left behind, or TODO without an owner and issue link.
• No unrelated formatting churn that would obscure the diff.
• N/A- Public API changes (if any) are intentional, documented, and reflected in affected callers/tests.

General Code Hygiene

• The code is self-explanatory; comments were added only where the intent or rationale is non-obvious (CONTRIBUTING.md §Code/General).
• Every modified or added file ends with a single trailing newline (CONTRIBUTING.md §Code/General).
• No trailing whitespace, inconsistent indentation, or mixed formatting styles remain.
• Identifiers referenced in comments or error messages are wrapped in Markdown backticks (e.g. the `AllReduce` implementation) (CONTRIBUTING.md §Code/General).
• All comments and error messages are in English (CONTRIBUTING.md §Code/General).
• Comments and error messages are complete sentences — capitalized first letter, terminal punctuation — unless the language/framework convention says otherwise (CONTRIBUTING.md §Code/General; §Python).

C++ Specific (if C++ files changed)

• Code follows the Google C++ Style Guide strictly.
• clang-format (version 16, per .github/workflows/clang-format.yml) has been run against all modified applicable files; the diff is clean.
• No exceptions are thrown. Error paths use assert with messages that include at least __FILE__, __LINE__, and __func__ (CONTRIBUTING.md §C++).
• Error and warning message wording follows the LLVM Coding Standards (CONTRIBUTING.md §C++).
• Constructor initializer list order matches member declaration order (CONTRIBUTING.md §C++).
• Exactly one blank line between classes, between classes and functions, and between functions (CONTRIBUTING.md §C++).
• Exactly one blank line between members (functions and variables) within a class (CONTRIBUTING.md §C++).
• Exactly one blank line before and after the contents of a namespace (CONTRIBUTING.md §C++).

Python Specific (if Python files changed)

• N/A.

Testing

• All applicable example programs have been built and tested successfully on at least one supported heterogeneous cluster setup.

Build, CI, and Tooling

• New backends or devices have been added to auto-detection in CMakeLists.txt under if(AUTO_DETECT_DEVICES) or to if(AUTO_DETECT_BACKENDS) if applicable.
• Both CI workflows (clang-format.yml, ruff.yml) are green locally (or expected to be green on CI).

Documentation

• README.md, CONTRIBUTING.md, or inline docs updated when behavior, build flags, or developer workflow changed.
• Any user-visible breaking change is called out explicitly under "Summary" and in the commit/PR title with a ! or BREAKING CHANGE: footer.

Security and Safety

• No secrets, access tokens, internal URLs, customer data, or personal hardware identifiers have been committed.
• N/A- Third-party code is license-compatible and attributed.
• No unsafe pointer arithmetic, uninitialized reads, or missing bounds checks were introduced.