[Feature] Add separated model_args flag

[DzmitryPihulski]

--model_args : Will controls parameters passed to the model constructor. Accepts a string containing comma-separated keyword arguments to the model class of the format "arg1=val1,arg2=val2,...", such as, for example --model_args pretrained=EleutherAI/pythia-160m,dtype=float32. The list of model args will be passed to huggingface auto model class