Add Metal test job to CI test matrix

[aditvenk]

Stacked PRs:

• [metal] Auto-cap threadgroup size via CuteND/CuteFlattenedTileStrategy #1855
• Add backend-agnostic lane loop APIs to tile strategies #1798
• ->Add Metal test job to CI test matrix #1862

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Add Metal test job to CI test matrix

Add a macOS Metal entry to matrix.json (macos-m2-26, cpu runtime,
metal backend) and update test.yml to handle macOS:

• Gate apt-get on runner.os == 'Linux'
• Add cpu runtime branch for PyTorch install
• Add MPS availability check for metal backend
• Run full test suite for metal (most tests skip via requires_gpu)
• Skip grid range tests that need loop_index_expr

[oulgen]

You dont need this, above sets HELION_BACKEND=metal and that should cover this

[aditvenk]

It collects all tests in "." and I was worried that there might be imports which cannot be resolved in mac. Checking if any imports cannot be resolved...

[oulgen]

if there are imports that cannot be resolved, then they should be guarded by the backend. i.e. pallas backend cannot import triton, so all triton imports are guarded

[aditvenk]

You dont need this, above sets HELION_BACKEND=metal and that should cover this

@oulgen -- just to clarify:

1. are you suggesting to not add a conditional here? If I don't, by default, we try to run all tests in the folder (incl test_metal.py), but a bunch of tests fail to import triton on mac and several more fail to find CUDA and fail. We will need to touch-up these files to exit gracefully on mac?

Or, 2) are you suggesting to use HELION_BACKEND in the condition's expression?

[aditvenk]

I have done (2) in the recent commit. PTAL

[oulgen]

Each test file is guarded by @onlybackend decorator. So metal would only run metal tests. You dont need this if block at all. Try running locally HELION_BACKEND=metal pytest test and you'll see

[aditvenk]

Ah thanks for clarifying! Didn’t know that. Will try it and amend this PR

[aditvenk]

Modified as suggested.

I had to skip a few tests that fail on Metal due to some missing feature not yet added