Allow creating multiple contexts per model

[LoganDark]

This allows for parallel inference and I am preparing to support sequence mode using a method similar to this (creating a new kind of graph)

Yes it should be perfectly safe to mix multiple ggml contexts like this, I looked into the code and there is no requirement that graphs only have to be created from the current context :3

[saharNooby]

Looks almost good, but I think a separate test sohuld be added -- see comment above.

[LoganDark]

I see what is going on, one sec

[saharNooby]

@LoganDark LGTM now, but why it is closed?