[CuTeDSL] Fix prep_editable_install.py for split nvidia-cutlass-dsl wheels

[zkyue]

Fixes #3416.

Symptom

The CuTeDSL editable dev flow (python/CuTeDSL/prep_editable_install.py, then setup.sh --editable) produces an install that fails at import cutlass with ModuleNotFoundError: No module named 'cutlass._mlir'. The prep script prints "Setup completed successfully!" while its summary shows 0 lib files, 0 Python files copied.

Root cause

Since 4.4.0 the nvidia-cutlass-dsl wheel on PyPI is a ~10 KB metadata-only shim; the runtime libraries and python payload moved to nvidia-cutlass-dsl-libs-{base,core,cu12,cu13} wheels (declared via Requires-Dist, cu13 behind the cu13 extra), and the in-wheel payload directory was renamed python_packages/ → dsl_packages/. The script still runs pip download --no-deps nvidia-cutlass-dsl, so it gets only the shim, extracts nothing, and exits 0 anyway.

Fix

• Detect a metadata-only top-level wheel by content; wheels that carry their own payload (<= 4.3.x) are handled exactly as before.
• For split wheels, follow the Requires-Dist entries within the nvidia-cutlass-dsl-libs-* family transitively, restricted to one CUDA variant (--cu12 default / --cu13, matching setup.sh), and extract all of them.
• Merge every bundled python_packages/cutlass / dsl_packages/cutlass tree (the payload now spans several wheels) instead of only the first match.
• Raise a clear error (instead of reporting success) when no matching platform artifact exists or no runtime library / cutlass python payload was extracted.

Repro / verification

python -m venv v && . v/bin/activate && pip install -U pip
cd python/CuTeDSL
python prep_editable_install.py          # before: 0 files copied, exit 0; after: libcute_dsl_runtime.so + cutlass/_mlir vendored
CUTLASS_IR_BUILD_DIR=$PWD ./setup.sh --editable
python -c "import cutlass, cutlass.cute as cute; print(cutlass.__file__)"

Verified: 4.6.1 cu12 end-to-end (import + cute.compile of a trivial kernel, no GPU required), cu13 dependency resolution, unchanged behavior on the pre-split 4.3.5 wheel, and the new failure messages.

Related: #3204 attempted this earlier (now stale); this PR is confined to prep_editable_install.py, follows the shim's own Requires-Dist pins (so it keeps working when the companion set changes, e.g. the cu12/core additions in 4.6.x), and keeps the CUDA-variant choice consistent with setup.sh --cu12/--cu13.

[brandon-yujie-sun]

@zekunf-nv