Wrapper for Hugging Face models

Make a wrapper class for Hugging Face models. The class will initialise the model (and the tokeniser). Perform a forward pass on an input list of text. We need functions like `get_grad()` too, since some attacks require grads.

Reference: https://github.com/QData/TextAttack/blob/master/textattack/models/wrappers/huggingface_model_wrapper.py