Please provide class-agnostic trained Mask-RCNN models in Model-Zoo

## 🚀 Feature
Please provide some pre-trained weights for Mask-RCNN model which are trained in a class-agnostic way, there are no weights for that at the moment.

## Motivation & Examples

It is useful for research in open-set/open-world object detection.

Start with the Model-Zoo and its existing Mask-RCNN model entries to understand how pre-trained weights are published and described. The issue does not name specific files or tests, so first locate the relevant model and training configuration entry points. Done means class-agnostic Mask-RCNN weights are available in the Model-Zoo with clear usage details.

Written by the indexing model from the issue text.