llama : add ability to load model from memory buffer

[ngxson]

TODO: add some tests

Currently, loading a model depends on file system. That means the model must firstly be saved onto disk, then being loaded using llama_load_model_from_file

However, when compiled to web assembly, the virtual fs implementation is very inefficient and does not support files bigger than 2GB. Allow loading file directly from a buffer will be bypass the need of fs.

This PR introduce 2 new APIs:

• gguf_init_from_buffer ==> this was left as a TODO, so I implemented it
• llama_load_model_from_buffers ==> support loading multiple model shards (or model splits)

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[sragrawal]

@ngxson Any update on this? This would be very useful for users who are downloading the model from an object store bucket into memory.

[ngxson]

@sragrawal sorry for the slow response. I don't think the mentioned use case can be benefit from the current PR.

Basically, you can make a tmpfs mount and download the file from object store into that tmpfs. With this, everything will be stored on RAM.

I think the use cases for this PR should be to support environments that have poor FS support, like wasm or sandboxed environment (i.e. iOS development). But these use cases are currently quite rare so I can't invest much time on that.