[RFC] Add MindSpeed/Megatron Unit Test CI Pipeline

## Background

ROLL has added more MindSpeed/Megatron-related functionality, including `mcore_adapter`, `roll/third_party/megatron`, Megatron offload, model update, checkpoint handling,and Ascend NPU runtime patches.

These components depend on MindSpeed, Megatron-Core, torch-npu, and the Ascend runtime. They are difficult to validate reliably through local tests or the existing CPU-only unit test pipeline.

This RFC proposes adding a dedicated MindSpeed/Megatron unit test CI pipeline to detect regressions earlier in PRs and main branch updates.

## Goals

Add a dedicated CI workflow for MindSpeed/Megatron unit tests that can validate:

- MindSpeed and Megatron-Core installation
- Basic NPU runtime availability
- ROLL Megatron adapter behavior
- Megatron offload-related unit tests
- NPU/Megatron runtime patch compatibility

## Non-Goals

This RFC does not aim to:

- Add full end-to-end training jobs
- Add performance benchmark jobs
- Run heavy NPU tests for every unrelated PR
- Replace the existing CPU unit tests or NPU integration tests

## Proposal

Add a new GitHub Actions workflow:

```text
.github/workflows/ci-npu-mindspeed.yml

Start by reviewing the existing workflows and the proposed .github/workflows/ci-npu-mindspeed.yml entry point, then identify how MindSpeed, Megatron-Core, torch-npu, and the Ascend runtime are handled. The work is done when a dedicated pipeline covers installation, basic NPU availability, adapter behavior, offload tests, and runtime patch compatibility without replacing CPU or integration pipelines.

Written by the indexing model from the issue text.