Add named manual kernel registration API

[goutamadwant]

Summary

Adds an opt-in library-name parameter for manual kernel registration codegen so generated RegisterKernels.{h,cpp} can expose a library-specific registration API such as register_portable_ops_lib_kernels().

The default manual registration API remains register_all_kernels() when no library name is provided. CMake manual-registration generation now passes the target library name through to codegen, and the Buck macro path supports the same optional parameter.

Fixes #11221.

Test plan

• PYTHONPATH=.. python3 -m unittest codegen.test.test_executorch_gen
• PYTHONPATH=.. python3 -m unittest codegen.test.test_executorch_gen codegen.test.test_executorch_signatures codegen.test.test_executorch_types codegen.test.test_executorch_unboxing codegen.test.test_selective_build
• python3 -m py_compile codegen/gen.py codegen/test/test_executorch_gen.py
• git diff --check origin/main..HEAD
• Ran a temporary codegen.gen --manual-registration --manual-registration-lib-name=portable_ops_lib invocation and verified generated RegisterKernels.h and RegisterKernelsEverything.cpp use register_portable_ops_lib_kernels().

Could not run CMake validation locally because cmake is not installed on this PATH.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20658

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⚠️ 11 Awaiting Approval

As of commit 5acef86 with merge base e17700a ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test Vulkan Backend (gh)
• Test WebGPU Backend (gh)
• Test XNNPACK Backend (gh)

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[goutamadwant]

@pytorchbot label "release notes: api"