[CUDA] Enable FP4 QMoE by default (#32096)
This change enables FP4 QMoE kernel instantiations by default for CUDA
builds. Non-CUDA builds remain disabled, and CUDA users can opt out with
`-Donnxruntime_USE_FP4_QMOE=OFF`.
The QMoE documentation now describes the default, dependency, and
override behavior.
## Validation
- `cmake -S /home/tianlei/git/onnxruntime/cmake -B
/tmp/ort_fp4_qmoe_check -Donnxruntime_USE_CUDA=OFF
-Donnxruntime_BUILD_UNIT_TESTS=OFF -Donnxruntime_BUILD_SHARED_LIB=OFF`
- `cmake -S /home/tianlei/git/onnxruntime/cmake -B
/tmp/ort_fp4_qmoe_cuda_check -Donnxruntime_USE_CUDA=ON
-Donnxruntime_BUILD_UNIT_TESTS=OFF -Donnxruntime_BUILD_SHARED_LIB=OFF
-Donnxruntime_USE_TENSORRT=OFF
-DCMAKE_CUDA_COMPILER=/home/tianlei/cuda13.0/bin/nvcc
-Donnxruntime_CUDNN_HOME=/home/tianlei/cudnn_9.23_cuda13`
- `git diff --check`
The CUDA configure cache records `onnxruntime_USE_FP4_QMOE:BOOL=ON`.
