fix: load HF model only on rank 0

[parthchadha]

What does this PR do ?

Loads model only on rank 0 and then uses fsdp2 set_model_state_dict API to load the weights on other ranks (after the model has been parallelized).
Note that the current PR still leads to GPU OOM for 70B model on 1 node with dtensor, will fix in a separate PR.

Issues

closes #279

Usage

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