Address CI build failure (#28243)
This pull request makes a small change to the CUDA label encoder kernel
to address unused parameter warnings. The change marks the `attr_name`
parameter as unused in the `TryGetScalarTensorAttribute` function when
building with the plugin execution provider.
* Code quality improvement:
* Marked the `attr_name` parameter as unused with
`ORT_UNUSED_PARAMETER(attr_name);` to suppress compiler warnings when
building with `BUILD_CUDA_EP_AS_PLUGIN`.
