Refactor Models builders to make them reusable on Prototype

[datumbox]

🚀 The feature

The current model builders are hard to reuse from the prototype area, leading to large amounts of copy-pasting. It would be good to refactor the space in a BC manner so that we can reuse as many parts as we can:

• Segmentation models:
  • Update them to follow the standard convention of defining the builders on the same file as their class.
  • Make private model builders reusable from prototype.
  • Clean up the implementations and expose publicly variables that are currently pushed to kwargs.
• Detection models:
  • Make private model builders reusable from prototype.
  • Refactor the VGG16 Features weights mechanism on SSD to work well with the proposed weights prototype.