Register buffer init callbacks in llama
Author: kwen2501
Pull request: yes

# What does this PR do?

Non-persistent buffers is not saved in state dict. 

In the case of meta init, while loading state dict from checkpoint can fill in parameters and persistent buffers, we need a way to initialize non-persistent buffers.

This PR does so by registering a buffer's init function against the buffer's FQN, and attaching such a callback dict to the model.

For how the init callbacks can be used, please refer to the `init_buffers` utility in this PR:
https://github.com/pytorch/PiPPy/pull/1135


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Comments:
Hi @kwen2501, thanks for opening this PR! 

cc'ing @muellerzr as this likely overlaps with accelerate's handling of weights loading and @ArthurZucker re Llama

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