tvm: decompose reduction initializers after scheduling

[guillon]

Motivation

TVM 0.26 keeps reduction initialization inside the update loop, which can prevent LLVM from automatically vectorizing otherwise vectorizable loops and cause significant performance regressions.

Description

Add a final TVM schedule transform that separates reduction initialization from update loops.

Use TVM's decompose_reduction primitive for regular reductions and a structural fallback for reductions hidden below XTC loop-partition scopes. Preserve spatial loop annotations, support cache-write buffers, and leave already externalized reductions unchanged because they no longer contain an initialization statement.

Emit the transform directly in generated TVM schedule fragments and update the corresponding FileCheck expectations.

Commits

• tvm: decompose reduction initializers

Discussion

This PR is stacked on #142, which itself depends on #133.

TODO:

• Merge build: use xtc-build for host artifacts #142 before this PR.

Testing

• uv run pytest tests/pytest/tvm -q — 11 passed
• uv run lit -j 8 tests/filecheck/backends tests/filecheck/schedules tests/filecheck/search — 113 passed, 3 expected failures, 24 unsupported
• Ruff, Pyright, and Mypy checks on the changed TVM transform code
• Numerical validation of partitioned reductions with a cache-write buffer
• make check-format currently reports unrelated pre-existing formatting issues in MlirOps.py and TVMOps.py

[guillon]

For information @liamsemeria, update such that with TVM 0.26 we hoist 0-initialization correctly.

[codecov]

Codecov Report

❌ Patch coverage is 86.27451% with 49 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/xtc/backends/tvm/TVMScheduleTransforms.py	89.02%	12 Missing and 15 partials ⚠️
src/xtc/schedules/loop_nest.py	52.63%	5 Missing and 4 partials ⚠️
src/xtc/backends/tvm/TVMCompiler.py	82.60%	4 Missing and 4 partials ⚠️
src/xtc/targets/host/HostAREvaluator.py	78.57%	2 Missing and 1 partial ⚠️
src/xtc/itf/schd/scheduler.py	50.00%	1 Missing ⚠️
src/xtc/targets/host/HostCEvaluator.py	93.33%	1 Missing ⚠️

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