Add automatic checkpoint downloading in Python API

[rasbt]

Adds the following behavior

a) Initialize a random model (no pretrained weights)

LLM(model="microsoft/phi-2", init="random")

b) DEFAULT: Download a model from the hub , but don’t download if a model exists locally via the "microsoft/phi-2" path:

LLM(model="microsoft/phi-2")

c) Load a model from a local directory:

Same as above, if model exists via the "microsoft/phi-2" path:

LLM(model="microsoft/phi-2")

d) Pro user can optionally suppress automatic download

LLM(model="microsoft/phi-2", init="local")

CC @lantiga @williamFalcon