refactor: Refactor relationship between non-hybrid classes and hybrid impl to use mixins

The challenge here is to give both the non-hybrid classes (llm_build_mamba
and llm_build_granite) AND the hybrid class (llm_build_hybrid_mamba) access
to the same intermediate "base class" functionality (build_mamba*_layer,
build_granite_attention_layer) without running into trouble with diamond
inheritance of llm_graph_context. Due to the non-trivial initialization
that happens in llm_graph_context, diamond inheritance results in multiple
initializations of the common base which cause problems around the unique
ptrs. I wanted to get away from `self->` everywhere, but this is still a
bit cleaner than making those methods static I think.

Branch: GraniteFour

Signed-off-by: Gabe Goodhart <[email protected]>

Author: gabe-l-hart