Update on "[Excutorch][Llama] Decouple input sequence length from kv cache context length"

Decouple max sequence length, for shape dynamism in torch.export, from sequence
length used for kv cache sizing.

Differential Revision: [D68448334](https://our.internmc.facebook.com/intern/diff/D68448334/)

cc mergennachin cccclai helunwencser dvorjackz

[ghstack-poisoned]

Author: kimishpatel