MPS support

[aszokalski]

Added lucent.util.set_default_device(backend_name) function to allow working with a mps backend.

NOTE: pytorch doesn't support all the required functions in the mps backend. You have to set an env variable PYTORCH_ENABLE_MPS_FALLBACK=1 to allow cpu fallback

Usage example:

from lucent.modelzoo import inceptionv1
import lucent.util as ut

device = ut.set_default_device("mps") # or cuda or cpu
model = inceptionv1(pretrained=True)
model.to(device).eval()