Speed up model loading by 25%

[daulet]

Load checkpoint directly to device, in my testing loading llama 7B went from 7.83 to 5.78 seconds (about 25% faster). I also noticed that memory usage doubles temporarily, at least until compilation finishes. My understanding is that it's due to storing the checkpoint in float16, so at loading time when we mmap it and assign we still have to cast them to bfloat16, which probably results in more allocation. Hence the change to convert_hf_checkpointto store weights in bfloat16 which is what is used by in generate by default. After re-exporting .pth files I no longer observed temporary 2x spike in GPU memory.

Also fix #22.

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