Safetensors: unpacking and packing

Safetensors can be unpacked, manipulated separately on tensors and metadata, and packed back up, just like an archive.

Packer usage:

python sft_pack.py [-h] [-o OUTPUT] input

where input is safetensors file, and output is directory name (optional).

Unpacker usage:

sft_unpack.py [-h] input

where input is directory structure