Speed up loading remote checkpoints

[Achal Dave]

open_lm/open_lm/file_utils.py

Line 70 in 9ca7042

 def pt_load(file_path, map_location=None): 

fsspec is somehow really slow at loading large files in my experience, and right now we have every process reading from s3. This is quite slow at large model sizes; it would be nice to speed this up, probably via subprocess.run("aws s3 cp ...") in local_rank=0 and then loading locally from each worker.