Adding instructions for generating model accuracy (#2855)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2855

Title

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: kimishpatel, lucylq

Differential Revision: D55775529

fbshipit-source-id: e3060dd7c1446413a5540b708b9374dd7a6b79c9