ggml : remove ggml_cplan + rework ggml_cgraph

[ggerganov]

target #9408

• Merge ggml_cplan into ggml_cgraph
• Rework the ggml_cgraph API
• Add comments indicating the CPU-specific ggml_cgraph API

I am thinking this could be a step towards removing the CPU-specific ggml interface and aligned with the idea from ggerganov/ggml#949 (comment)

[slaren]

I am not sure if I understand the purpose of these changes. Mainly it seems to replace the CPU backend cplan API with another set of functions that are also specific to the CPU backend, but it also moves the data previously stored in ggml_cplan to ggml_cgraph, effectively increasing the coupling between the CPU backend and the common ggml code.

[ggerganov]

The main goal was to make struct ggml_cplan private, but couldn't find a simple way to avoid the return by value of ggml_graph_plan, so decided to merge it in ggml_cgraph. It does increase the coupling so in this regard it might not be worth to make this change.

[slaren]

If the goal is to access the CPU backend through ggml-backend, then the CPU backend functions (including ggml_cplan) can be made private, and only the ggml-backend interface to the CPU backend (ggml_backend_cpu_init, ggml_backend_cpu_set_n_threads, etc) would need to be exposed.

[ggerganov]

Right, that would be the better approach. We can do this change at some point to simplify the API. Not high priority though.