Modules registry for supporting custom backbone networks

[arhanjain]

Addresses #189

The implementation consists of adding a registry to the Module class so that all child classes are registered by their name into the registry upon __init_subclass__. This allows one to initialized backbones (like in parameters of VisualCore) through the registry rather than using eval which only allows models from base_nets.py

Example script coming soon

[danfeiX]

Thanks! I think it'd be much better to move this to EncoderCore in obs_core.py, as many classes that inherits Module are not necessarily encoders

[arhanjain]

looks like there's already a encoder registry, so we just need to modify the way the VisualCore class initializes its backbone!

[amandlek]

Thanks @arhanjain for your contribution!