Add llama animated gif to llama readme (#5474)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5474

bypass-github-export-checks
bypass-github-pytorch-ci-checks
bypass-github-executorch-ci-checks
allow-large-files

Reviewed By: lucylq, guangy10

Differential Revision: D62968545

fbshipit-source-id: da7554ba40f2460adfd4ea178431610e8b831587