[Enh] Improve macro name robustness

[sjfeng1999]

Motivation

Technical Details

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

[Copilot]

Pull request overview

This PR refactors how FlyDSL iterates over enabled backend stacks at compile time by replacing the FLYDSL_BACKENDS_TUPLE-based preprocessor iteration with a more robust FLYDSL_BACKEND_COUNT + FLYDSL_BACKEND_0..N-1 scheme generated by CMake.

Changes:

• Introduces include/flydsl/Backend/Backend.h providing FLYDSL_FOR_EACH_BACKEND(...) driven by FLYDSL_BACKEND_COUNT.
• Updates CMake backend configuration (cmake/FlyDSLBackends.cmake) to emit FLYDSL_BACKEND_COUNT and per-index backend defines instead of FLYDSL_BACKENDS_TUPLE.
• Updates backend registration call sites (fly-opt and Python registration) and adjusts the unit test accordingly; removes the old ForEachBackend.h.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tools/fly-opt/fly-opt.cpp	Switches backend iteration/registration to FLYDSL_FOR_EACH_BACKEND.
python/mlir_flydsl/FlyRegisterEverything.cpp	Switches Python-side backend registration to FLYDSL_FOR_EACH_BACKEND.
cmake/FlyDSLBackends.cmake	Emits FLYDSL_BACKEND_COUNT and FLYDSL_BACKEND_<i> defines; adds max-backend guard.
include/flydsl/Backend/Backend.h	New preprocessor iteration utility for backends.
include/flydsl/Backend/ForEachBackend.h	Removes the old tuple-based backend iteration utility.
tests/unit/test_backend_cmake_defaults.py	Updates guardrail assertions to match new CMake compile definitions.

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