Add example of LLava encoder (#2375)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2375

Add the example to start enabling LLava, one multimodal model in generative AI area. In this example, we initiate the process of running LLava through ExecuTorch. Refer to the added README.md for details.

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Reviewed By: cccclai

Differential Revision: D54812717

fbshipit-source-id: 57f79a925f40594d6c0714b77aefb6193ee2890a