[metal] Reuse Inductor's MetalOverrides for MSL expression emission

[aditvenk]

Stacked PRs:

• [metal] Auto-cap threadgroup size via CuteND/CuteFlattenedTileStrategy #1855
• Add backend-agnostic lane loop APIs to tile strategies #1798
• [metal] Add elementwise kernel tests #1796
• Add Metal test job to CI test matrix #1862
• [metal] Add MSL AST walker for Python-to-C++ translation #1794
• Add Backend.post_process_function_def hook #1851
• [metal] Add Metal codegen handlers for load, store, and mask_to #1854
• ->[metal] Reuse Inductor's MetalOverrides for MSL expression emission #1853

---

[metal] Reuse Inductor's MetalOverrides for MSL expression emission

Subclass Inductor's torch._inductor.codegen.mps.MetalOverrides instead
of reimplementing ~170 lines of math/cast/comparison overrides. This
brings NaN-safe math (c10::metal::max/min), IEEE-compliant precision
(metal::precise::*), and type-safe casts (decltype) for free.

The C++ namespace syntax (::) in Inductor's expression output is
replaced with . before Python AST parsing, then restored to :: by the
MSL walker. Override where() to use Python ternary (parseable by AST),
and _special_unary/_special_binary to skip V.kernel.headers dependency.

[jansel]

Can you reuse the version of this in inductor:
https://github.com/pytorch/pytorch/blob/47fa87a257f72a30c2028e1200c1673ed36125b8/torch/_inductor/codegen/mps.py#L185

[aditvenk]

Can you reuse the version of this in inductor: https://github.com/pytorch/pytorch/blob/47fa87a257f72a30c2028e1200c1673ed36125b8/torch/_inductor/codegen/mps.py#L185

Will take a look at it. From skimming it seems like it pulls in Inductor dependencies (V.kernel, CSEVariable) and also emits MSL differently.

[aditvenk]

Can you reuse the version of this in inductor: https://github.com/pytorch/pytorch/blob/47fa87a257f72a30c2028e1200c1673ed36125b8/torch/_inductor/codegen/mps.py#L185

Refactored to use Inductor's override for 99%. A couple of methods need to be handled slightly differently because Helion parses override strings as Python AST. While this works completely for Triton/Cute/Pallas because those are valid python syntax. For Metal, since it is C++, some things like ::, C++ ternary operator don't parse.