[Excutorch][Llama] Decouple input sequence length from kv cache context length (#8047)

Pull Request resolved: https://github.com/pytorch/executorch/pull/7927

Decouple max sequence length, for shape dynamism in torch.export, from sequence
length used for kv cache sizing.
ghstack-source-id: 263653316

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

Co-authored-by: Kimish Patel <kimishpatel@fb.com>