Set cb_eval after context creation

[vladfaust]

I'd like to be able to change cb_eval and cb_eval_user_data prior to calling llama_decode, per-call, to track decoding progress with different callbacks. Currently it's a part of llama_context.cparams. Exposing the whole .cparams field doesn't seem to be viable, but changing these two is required in my use-case. Any workarounds?

[ggerganov]

You can have a wrapper callback that calls the callback that you want