Serialize and save model when initializing workspace

[ento]

Motivation

deploy.load_model takes an input shape argument, which is hardcoded in bot/webapp. This value is not something dependent on the code, but rather tied to the model that goes with the saved weights. Furthermore, the saved weights are tightly coupled with the model itself.

Coupled data should saved in proximity with each other, not updated/specified by human hands.

Proposed changes

When initializing a workspace, user can specify the model builder name and input/output shapes. Build a model and save it in the workspace.

All other code should load the model from the workspace, and get input/output shapes from the model.