CUDA: fix build error from ambiguous __half conversions in conv2d (#15690)

CUDA: fix build error from ambiguous __half conversions in conv2d (#15690)

* CUDA: fix build error from ambiguous __half conversions in conv2d

Building conv2d with half precision failed because `__half` defines
multiple implicit conversion operators (to float, int, short, etc.),
causing ambiguous overload resolution when multiplying with float.

Introduce a templated `to_float` helper that explicitly converts
`__half` via `__half2float`, while passing through float unchanged.
Use this helper in conv2d accumulation to ensure unambiguous and
correct promotion to float.

Fixes some build errors with half-precision kernels on CUDA.

ggml-ci

* CUDA: Replace custom to_float helper with unified ggml_cuda_cast and add half‑>float conversion

* CUDA: Add missing convert.cuh header

* CUDA: remove unnecessary extension in ggml_cuda_cast

* CUDA: Address review comment, remove second type template argument