Safetensors offload
# What does this PR do?

This PRs make offload to disk more efficient for models that have a checkpoint in safetensors format: instead of re-saving everything as Numpy memory-mapped array, it uses directly the fact we can access the tensor in the checkpoint without loading the rest.

Goes with https://github.com/huggingface/accelerate/pull/873
_The documentation is not available anymore as the PR was closed or merged._