off by one error in sdpa cache op (#2689)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2689

start_pos is where we currently are. so when start_pos = 127 we are currently generating the 128th token

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Reviewed By: kimishpatel

Differential Revision: D55370500

fbshipit-source-id: 412b1638cc320a4df56dcc38995f3836ed5c425b