[Executorch][llama] Change runner to decouple prompt length from sequence length

[kimishpatel]

Stack from ghstack (oldest at bottom):

• -> [Executorch][llama] Change runner to decouple prompt length from sequence length #9350
• [Executorch][SDPA] Fix bug in sdpa #9105

length

Following previous diff now we can utilize entire kv cache to generate more
tokens than max prompt length allowed.

Differential Revision: D69073908

[pytorch-bot]

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[github-actions]

Looks like this PR hasn't been updated in a while so we're going to go ahead and mark this as Stale.
Feel free to remove the Stale label if you feel this was a mistake.
If you are unable to remove the Stale label please contact a maintainer in order to do so.
If you want the bot to never mark this PR stale again, add the no-stale label.
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