Models and Metrics for Molecular Generation

I am going to implement some of the generative models (VAE, AAE, ...) for molecular generation in deepchem. For this purpose, I think [moses](https://github.com/molecularsets/moses) library can be a good starting point. They have benchmarked different generative models and provided a set of metrics to evaluate the quality of generated molecules.

I think at first, I will try to add one or two models to the deepchem library. Moreover, I am going to add most of the metrics for evaluating quality.

I have started this issue to track my progress along this path.