Refactor: rename `n_swa` to `n_aux`

[ngxson]

Background Description

Ref: #12791 (comment)

This won't be a simple renaming, but we also need to update some logic:

I'll do this after having the logits matching automated test. The problem is that changing name n_swa to n_pattern_aux make the code checking if (n_swa) then using SWA becomes invalid. It should now become if (n_pattern_aux && is_swa) then use SWA

I think it's better to add an enum called llama_mask_aux having 3 values: none, swa, chunked ; so that the code will become more clear

Possible Refactor Approaches

Add enum llama_mask_aux

[ggerganov]

Btw, I was thinking a bit more about this, and SWA seems to fit both the regular SWA and the chunked Gemma-style attention. The latter is in some sense also "sliding", but in bigger "steps". So probably if we don't encounter more conceptually different use cases that make use of the n_swa functionality, it's OK to keep calling it like this.

[ngxson]

Hmm yeah that make sense then, let's keep n_swa then