[Build] Fix cuda packaging build error (#29580)
### Description
Fixes the CUDA packaging pipeline build error (with CUDA arch `52-real`)
that was introduced by #29451.
### Root cause
The small-M batched GEMV path added in #29451 defines `half` helpers for
the batched kernel — the `Acc2<half>` accumulator trait and the
`PackNatural` / `DotAccum` / `HorizontalAdd` overloads — inside a single
`#if (!defined(__CUDA_ARCH__) || __CUDA_ARCH__ >= 530)` guard that
compiles the *entire* declaration out for archs below `sm_53`.
However, `MatMulFloat4BatchedKernel<half>` is launched from host code
and is therefore instantiated for **every** target architecture,
including `sm_52`. During the `sm_52` device pass those `half` helpers
no longer exist, so `nvcc` reported (100 errors, e.g.):
- `incomplete type "Acc2<half>" is not allowed`
- `no suitable user-defined conversion from "WPack<half>" to "const
WPack<nv_bfloat16>" exists` (the compiler fell back to the `nv_bfloat16`
overloads)
### Fix
Mirror the existing `nv_bfloat16` convention in the same file: always
define the type and function **signatures**, and gate only the
arch-specific (half2-intrinsic) **bodies**. Now `Acc2<half>` is always a
complete type and the `half` overloads always exist, so
`MatMulFloat4BatchedKernel<half>` compiles for `sm_52`. On archs below
`sm_53` the bodies compile to no-ops (returning zeros), which matches
the already-shipped `nv_bfloat16` behavior for archs below `sm_80`.
### Motivation and Context
The CUDA packaging pipeline builds with
`CMAKE_CUDA_ARCHITECTURES=52-real;61-real;75-real;86-real;89-real;90-virtual`,
so the `sm_52` device pass is required and the pipeline was broken by
#29451.
