[find_people] Detector replacement (SSD Keras -> PyTorch Faster R-CNN)

[alex-mitrevski]

Summary of changes

This PR replaces the SSD Keras model used in the find_people action with the PyTorch-based Faster R-CNN ResNet-50 FPN model. This is part of an effort to replace all Tensorflow-based models in our code base with PyTorch-based models (which already started with https://github.com/b-it-bots/mas_domestic_robotics/pull/207).

Related PRs:

• https://github.com/b-it-bots/mas_perception_libs/pull/21
• https://github.com/b-it-bots/mas_perception_libs/pull/22

Tests to verify the changes

I performed tests with Lucy for verifying the changes.

cc @AhmedFaisal95