Add the option to finetune a policy on an MPS device

[marinabar]

What this does

Adds the option to finetune a policy on a new device (MacBook MPS).

This way the weights are loaded from the safetensors file.

The YAML config is copied from the finetuned_policy_path, which should be passed in via +finetune=pretrained_policy_path

How it was tested

python lerobot/scripts/train.py +finetune=m1b/act_koch_pick_lego_green_box

How to checkout & try? (for the reviewer)

python lerobot/scripts/train.py +finetune=lerobot/act_aloha_sim_transfer_cube_human