[CICD] Add KunlunXin unit and MCore integration test support

[AlexMa616]

Description

Summary

This PR adds dedicated KunlunXin CI support for TransformerEngine-FL using the existing vendor.kunlunxin backend.

Changes

• Added the KunlunXin platform configuration, environment setup script, and test workflow.
• Added KunlunXin launchers for:
  • PyTorch debug tests
  • PyTorch unit tests
  • Distributed tests
  • ONNX export tests
  • MCore integration tests
• Added an explicit setup check to ensure:
  • transformer_engine_klx_torch can be imported.
  • The KunlunXin backend is available.
  • generic_gemm selects vendor.kunlunxin instead of falling back to another backend.
• Kept KunlunXin-specific environment handling and compatibility logic outside the common workflows.
• Added generic MCore entrypoint and CUDA-default controls for platform-specific launchers.
• Pinned the Megatron-LM-FL revision used by the MCore integration test.
• Enabled coverage collection and aggregation.

No TransformerEngine core source, upstream test cases, other platform configurations, or third-party submodules are changed.

Testing

The previous self-hosted KunlunXin workflow run passed:

• PyTorch debug tests
• PyTorch unit tests
• Distributed tests
• ONNX export tests
• Coverage aggregation
• MCore integration tests

The final cleaned branch additionally passed shell syntax, YAML parsing, embedded Python syntax, configuration path, and diff consistency checks.