Subclass the PyTorchDetectionTransformer estimator off the PyTorchObjectDetector estimator

[f4str]

Is your feature request related to a problem? Please describe.
There is a lot of code overlap between the PyTorchDetectionTransformer and PyTorchObjectDetector estimators. This causes bugs to have to be fixed in both locations when reported. Missed changes in one estimator will still remain a bug in the other. In additional, the big overlap in code is redundant and makes it tedious from a maintenance perspective.

Describe the solution you'd like
The PyTorchDetectionTransformer estimator should be subclassed off PyTorchObjectDetector. This will reuse a lot of the code and avoid redundant bugs.

Describe alternatives you've considered
N/A

Additional context
This is purely an codebase housekeeping change. Functionality should not be changed in any way.

[beat-buesser]

This issue is very closely related to #2267. I think it is better to make both changes in the same pull request.