mtmd: add mtmd_context_params::warmup option

[ngxson]

Supersede #17638

If user wants to warmup with a different image size, set warmup = false, and send one image with the desired size right after mtmd initialized for manual warmup

warmup = false can be set via the existing --no-warmup CLI argument

[ggerganov]

This is probably more like a "reserve worst case" rather than a "warmup" if I understand correctly.

Does the warmup in CLIP do any actual ggml graph computation?

[ngxson]

Does the warmup in CLIP do any actual ggml graph computation?

It doesn't run the computation, but it allocate enough memory for cgraph.

For context: Ideally, the memory will be allocated for the worst case (max image size), but this doesn't work very well as some models support maximum image size that worth up several GB of memory for cgraph. Therefore, we have to set an arbitrary "ideal case" for this, hence set_warmup_n_tokens function.

This was done mostly to improve UX as we don't either want to limit the max image size to an arbitrary number.But we should improve this in the future.

Also, during the warmup, we also scan the cgraph for ops that are not supported by GPU.