feat(hiptensor): Build lib with a stub implementation when GPU_TARGETS is empty

[evedovelli]

Motivation

hipTensor depends on Composable Kernel, which only supports a subset of gfx targets. On architectures where the effective GPU_TARGETS resolves to empty (e.g. a single-arch build for an unsupported gfx), the device library and its CK
dependency cannot be built, which previously broke configuration and left downstream consumers with no hiptensor package at all.

This PR makes hipTensor degrade gracefully in that case by building a host-only stub library

Technical Details

• HIPTENSOR_ENABLE_DEVICE option: when GPU_TARGETS is explicitly empty, it is turned off automatically (with a warning) instead of failing the build.
• Stub library (library/stub/): a host-only libhiptensor that implements the full public API, returning HIPTENSOR_STATUS_NOT_SUPPORTED for every entry point. The package, headers, and hiptensor-config.cmake are still produced, so
  downstream find_package(hiptensor) and linking keep working, and callers get a well-defined runtime error instead of an unresolved symbol.
• Device build unchanged: when GPU_TARGETS is valid, the real library/src library is built as before; CK and tests/samples only configure in that path.
• Build-time API coverage check: a configure-time codegen parses the public header for every HIPTENSOR_EXPORT function and links a reference to each against the stub. If the stub is missing any public API, the build fails with an
  unresolved symbol. This runs for any GPU_TARGETS (device or stub), so the stub can never drift out of sync with the public API.

Test Plan and Results

• Device build (GPU_TARGETS=gfx1201, HIPTENSOR_ENABLE_DEVICE=ON): full build + coverage check link, PASS.
• Stub build (empty GPU_TARGETS): stub libhiptensor, headers, and CMake config produced and installed; coverage check links, PASS.
• Negative: removing a stub function fails the build with undefined symbol: hiptensorReduce, confirming the coverage check catches an incomplete stub.

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

ISSUE ID : ROCm/TheRock#6268

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[TorreZuk]

The stub pattern seems fine but does add maintenance for for matching up change in API.
Not clear if some macro stubs could also be used in the original API to comment out the code blocks and just return not_supported but that one might think is quirky as well.

[evedovelli]

The stub pattern seems fine but does add maintenance for for matching up change in API. Not clear if some macro stubs could also be used in the original API to comment out the code blocks and just return not_supported but that one might think is quirky as well.

Thank you for the review @TorreZuk. I'll dig and see if I could put something like this in place. For now there is an extra maintenance in the cpp stubs, but I did put in place a compile time verification that will trigger a failure if the stub implementation isn't aligned with the API.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

❌ Your project status has failed because the head coverage (76.92%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8874      +/-   ##
===========================================
+ Coverage    65.13%   65.17%   +0.05%     
===========================================
  Files         2628     2628              
  Lines       413118   413284     +166     
  Branches     61884    61891       +7     
===========================================
+ Hits        269044   269348     +304     
+ Misses      123563   123391     -172     
- Partials     20511    20545      +34     

Flag	Coverage Δ		*Carryforward flag
TensileLite	33.42% <ø> (ø)		Carriedforward from 4da814d
hipBLAS	90.81% <ø> (ø)		Carriedforward from 4da814d
hipBLASLt	41.36% <ø> (ø)		Carriedforward from 4da814d
hipCUB	82.68% <ø> (ø)		Carriedforward from 4da814d
hipDNN	85.91% <ø> (ø)		Carriedforward from 4da814d
hipFFT	50.17% <ø> (ø)		Carriedforward from 4da814d
hipRAND	76.12% <ø> (ø)		Carriedforward from 4da814d
hipSOLVER	69.18% <ø> (ø)		Carriedforward from 4da814d
hipSPARSE	86.55% <ø> (ø)		Carriedforward from 4da814d
rocBLAS	48.49% <ø> (+0.43%)	⬆️
rocFFT	46.30% <ø> (ø)		Carriedforward from 4da814d
rocRAND	57.04% <ø> (ø)		Carriedforward from 4da814d
rocSOLVER	76.92% <ø> (ø)		Carriedforward from 4da814d
rocSPARSE	72.37% <ø> (ø)		Carriedforward from 4da814d
rocThrust	91.36% <ø> (ø)		Carriedforward from 4da814d

*This pull request uses carry forward flags. Click here to find out more.
see 35 files with indirect coverage changes

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