Improve CTD preprocessors pipeline - #2984

This Pull request

1. Adds a processing stage to the CTD preprocessors that filters out empty bounding boxes and corresponding poses. This prevents the CTD inference to crash when the BU provides a pose with 0 or 1 found bodypart, which was leading to trying to top-down crop using an empty bbox.
2. Adds a processing stage to filter out bounding boxes that are below a certain confidence level.
3. Adds unit test to test these stages.
4. Silences Runtime warning occuring during the computation of a bbox from an empty pose.