Atrous convolutions for biologics

Linking discussion from https://github.com/deepchem/deepchem/issues/616

Regular graph convolutions only draw in information from an atom's direct neighbors. Atrous graph convolutions could draw in information from an atom's bond-distance 2 or 3 neighbors. This allows the receptive fields of graph convolutions to grow much larger. Note that a 4 layer deep graph convolution has visibility 4 bonds out, while a 4 layer deep 2-atrous graph convolution has visibilty 2^4=16 bonds out. This long range visibility could be a key enabler for working with biologics, which are much bigger molecules (protein antibodies typically) than small molecules.

PRs for atrous graph convolutions either here or in deepchem/deepchem welcome!