Implement `get_activations()` for PyTorch Object Detectors

[Farhan Ahmed]

Is your feature request related to a problem? Please describe.
The current PyTorch object detectors (PyTorchObjectDetector, PyTorchFasterRCNN, PyTorchYolo, PyTorchDetectionTransformer) do not implement the get_activations() method and therefore cannot extract activations.

Describe the solution you'd like
Using the model wrapper method used by HuggingFaceClassifierPyTorch, support can be added for activations for the PyTorch object detectors. This will allow usage of activation-based defenses such as Activation Clustering and Spectral Signatures.

Describe alternatives you've considered
N/A

Additional context
This will be worked on after #2267 is completed to avoid redundant code.

[Beat Buesser]

Hi @f4str Sounds good!