Simplify existing models

[cthoyt]

This pull request simplifies the three models by doing the following:

1. Switch to using the module interface instead of the functional interface for elements like ReLU, sigmoid, and dropout
2. Use nn.Sequential() to reduce stacked calls to modules/functions. This has the added benefit of removing confusing nomenclature
3. Group related functionality into helper functions (e.g., when the same thing is called on the left molecule and right molecule)
4. Improve documentation

[benedekrozemberczki]

Looks good! Definitely cleaner!