[Example] ggml: Update llama README for llama3 support

[hydai]

No description provided.

[juntao]

Hello, I am a code review bot on flows.network. Here are my reviews of code commits in this PR.

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Commit 298174dbf08ae9c472e054cbac9c37a2768ed375

Key Changes:

1. Updated the README files for llama-stream and llama in the wasmedge-ggml repository to support llama 3.
2. Added instructions for downloading the model file Meta-Llama-3-8B-Instruct.Q5_K_M.gguf.
3. Provided execution commands for running the models with specific environment variables such as llama3 and n_gpu_layers.
4. Included details for changing the context window size and running the models with different configurations.

Potential Problems:

1. The patch appears to be adding support for llama 3 alongside existing support for llama 2, but there might be conflicts if both versions are not properly handled.
2. The instructions provided in the README files seem clear, but it would be beneficial to verify that the changes are accurate and the commands are functioning as intended.
3. The patch introduces many new lines of code, so it's important to review for any inconsistencies with the existing codebase or documentation style.
4. There could be potential issues related to dependencies or compatibility that arise when users try to run the models following the updated instructions.

Overall, the changes aim to enhance the llama models' usability, but thorough testing and validation are necessary to ensure the new features work correctly.