Visualization bug fixes

[sovrasov]

What does this PR do?

• add missing None checks on saliency_maps arrtibute
• fix changing colors when launching instance segmentation visualization multiple times
• add render() method to Visualizer

Fixes # (issue)

Before submitting

• Did you write any new necessary tests?
• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?