Re-enable GPU assertions on RetinaNet unit-tests

## 🚀 Feature
Improve RetinaNet unit-test coverage and reenable all GPU assertions.

## Motivation

There is numerical/sort instability on the unit-tests of RetinaNet across platforms. PR #2812 disabled some of the checks on GPU as a workaround but this needs to be addressed.

## Pitch

Investigate the source of instability, fix it and re-enable all unit-tests in all platforms.