Ensemble faster-rcnn detectors

[sfmig]

Description

In this PR

• ensemble model for inference (supports detectors under torchvision.models.detection)
• module for fusing bboxes
• eval --- separately?
• example usage

References

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How has this PR been tested?

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Is this a breaking change?

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Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit