Registry refactor to resolve naming conflicts

[nmatthews-asapp]

Is your feature request related to a problem? Please describe.
Currently registering multiple classes of the same name with YAML will not work properly. We should allow multiple distinct classes of the same name to be registered in different namespaces, e.g. "NLLLoss" and "torch.NLLLoss"

Describe the solution you'd like
The registry should be a separate entity (we should not rely on ruamel.yaml to maintain the registry) so that we can easily manage these cases. The registry will be implemented to organize namespaces as a mapping for tags and classes. Whenever ruamel.yaml is needed, the registry can be synced with yaml to ensure it's up to date.

[nmatthews-asapp]

Fixed in v0.5 (refactor branch)