feat(metax): add graph capture and replay support

[gongchensu]

Summary

• Add MetaX graph capture and replay support by adapting the MC stream and graph APIs in src/native/cuda/metax/runtime_.h.
• Reuse the shared GraphRuntime implementation and adapt the five-argument mcGraphInstantiate signature.
• Add native MetaX graph coverage in tests/CMakeLists.txt and update docs/backends.md.

Motivation

MetaX MC runtime provides stream capture and graph execution APIs, but InfiniRT did not expose them through its graph runtime abstraction. This prevented downstream users such as InfiniCore and InfiniLM from using the InfiniRT graph capture and replay path on MetaX devices.

Related issue: N/A - this change is not associated with an existing issue.

Type of Change

• feat - new feature / new backend capability / new public API
• fix - bug fix
• perf - performance improvement without behavior change
• refactor - code restructuring without behavior change
• test - adding or fixing tests only
• docs - documentation only
• build / ci - build system or CI configuration
• chore - tooling, formatting, or other non-code changes
• Breaking change (requires a ! in the Conventional Commits prefix or a BREAKING CHANGE: footer)

Platforms Affected

• CPU (WITH_CPU)
• NVIDIA (WITH_NVIDIA)
• Iluvatar (WITH_ILUVATAR)
• Hygon (WITH_HYGON)
• MetaX (WITH_METAX)
• Moore (WITH_MOORE)
• Cambricon (WITH_CAMBRICON)
• Ascend (WITH_ASCEND)
• Build system / CMake / generated headers
• Public headers / installed consumer API
• Documentation only

Smoke Build and Test Result

paste smoke build and test output here

Test Results on Supported Platforms

Platform	Affected	Build / Smoke Result	Full Result / Notes
CPU
NVIDIA
Iluvatar
Hygon
MetaX
Moore
Cambricon
Ascend

Full `ctest` output (optional)

paste here

Benchmark / Performance Impact

Notes for Reviewers