llama : use n_swa + n_ubatch cells for SWA cache

[ggerganov]

target #13845

Overview

• SWA cache now uses less memory
• Enable SWA speculative decoding
• Allow short SWA rollbacks (avoids cache recalculations caused by whitespace truncation of the last response)

./scripts/compare-commits.sh master gg/swa-optimize -m models/gemma-3-4b/ggml-model-q8_0.gguf -d 8192 -p 0 -b 512,1024,2048,4096,8192 -n 32 -fa 0,1 -t 1

Model	Batch size	FA	Test	t/s master	t/s gg/swa-optimize	Speedup
gemma3 4B Q8_0	512	No	tg32@d8192	75.63	75.83	1.00
gemma3 4B Q8_0	512	Yes	tg32@d8192	75.12	75.13	1.00
gemma3 4B Q8_0	1024	No	tg32@d8192	73.68	76.03	1.03
gemma3 4B Q8_0	1024	Yes	tg32@d8192	74.94	75.15	1.00
gemma3 4B Q8_0	2048	No	tg32@d8192	69.73	75.92	1.09
gemma3 4B Q8_0	2048	Yes	tg32@d8192	74.62	75.14	1.01
gemma3 4B Q8_0	4096	No	tg32@d8192	62.43	75.92	1.22
gemma3 4B Q8_0	4096	Yes	tg32@d8192	74.39	75.15	1.01
gemma3 4B Q8_0	8192	No	tg32@d8192	54.37	76.00	1.40
gemma3 4B Q8_0	8192	Yes	tg32@d8192	73.25	75.11	1.03

[aviallon]

I'll try testing.
Edit: I got distracted, and forgot to test it. Oops.

[ngxson]

pos_min == -1 meaning the seq is empty. In this case, I think the behavior of setting n_past = 0 is expected, so we don't necessary need to log the warning

[ggerganov]

If the sequence is not present in the KV cache (i.e. pos_min == -1), but we somehow decided that slot.n_past > 0 (see the condition above) then this is still unexpected. I think we might even want to abort in such cases, because it means there is a bug somewhere.