FMHA examples: use cute::min in device functions

[LwhJesse]

Use cute::min instead of std::min in FMHA device functions.

These helpers are marked CUTLASS_DEVICE, and cute::min is already the CuTe/CUTLASS host-device constexpr utility used for this pattern.

This matches the existing CUTLASS/CuTe device-code style and avoids relying on std::min in device functions.

Testing:

cmake -S . -B build \
  -DCMAKE_BUILD_TYPE=Release \
  -DCUTLASS_NVCC_ARCHS=90a \
  -DCUTLASS_ENABLE_TESTS=OFF \
  -DCUTLASS_ENABLE_EXAMPLES=ON

cmake --build build --target 88_hopper_fmha -j$(nproc)

Result:

Built target 88_hopper_fmha

I did not build the Blackwell FMHA target locally.