Fix Prefast warning in CUDA contrib op (#14074)
### Description
Fixes Prefast C26814
```shell
onnxruntime::contrib::cuda::QAttention<onnxruntime::MLFloat16,signed char>::ComputeInternal
onnxruntime/contrib_ops/cuda/quantization/attention_quantization.cc
The const variable 'element_size' can be computed at compile-time. Consider using constexpr (con.5).
```
