[llama-mm] Enable kv cache for MultiHeadAttention

[pytorchbot]

This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #6793
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/larryliu0820/55/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/larryliu0820/55/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/larryliu0820/55/orig

[pytorch-bot]

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❌ 1 New Failure

As of commit ab90d92 with merge base 4947e27 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  adb: failed to install android-test-debug.apk: Failure [INSTALL_PARSE_FAILED_NOT_APK: Failed to parse /data/app/vmdl313180744.tmp/base.apk: Failed to load asset path /data/app/vmdl313180744.tmp/base.apk]

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