Add missing declarations for explicit instantiations in concat_lib and...

Add missing declarations for explicit instantiations in concat_lib and split_lib, and add new headers concat_lib_gpu.h and split_lib_gpu.h to contain them (and the declaration of the primary templates).

The current behaviour (using externally defined instantiations without having seen a declaration of those external instantiations) is undesirable and effectively deprecated, and is warned about by -Wundefined-func-template.

PiperOrigin-RevId: 237158146