kv-cache: Fix state restore fragmented cache

[ssweens]

Fix #17527

Change find_slot to allow non-contiguous allocation during state restore. Fixes 'failed to find available cells in kv cache' error when restoring state to fragmented cache.

[ssweens]

PS - aimed to implement the direction given by @ggerganov (#17527 (comment))

[ggerganov]

Looking good - will do some testing to confirm 👍

[ggerganov]

I updated the test to do a proper verification:

• It requires kv_unified == true
• It interleaves the sequences to create large fragmentation
• Uses total context size of 256 tokens and 3 sequences with 70 tokens each

@ssweens After you address the comments, we can merge.

[ssweens]

Thanks @ggerganov. Comments addressed and committed.

[ggerganov]

Thanks for the contribution. This is a good improvement.

[ssweens]

Happy to help where/when I can. Hat tip to you and team on the great project.