fix runner max seq len (#2688)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2688

Max seq len arg when not passed uses the max seq len from the model.
THis means, num tokens generated should be equal to kv cache size.
However, generate loop tries to generate one more token because pos, 0 based
index, is taken for num tokens

Reviewed By: mergennachin, digantdesai

Differential Revision: D55369776

fbshipit-source-id: 7beb38177a23449649e96184b0b0a0bb507c199f