It seems to me there is no feature extractor part exist in inception_v3 as it gives  the following error:
    backbone = backbone_models[BACKBONE_MODEL_NAME]['model'](pretrained=True).features
  File "/home/gpu-user/miniconda3/envs/faster_rcnn/lib/python3.8/site-packages/torch/nn/modules/module.py", line 778, in __getattr__
    raise ModuleAttributeError("'{}' object has no attribute '{}'".format(
torch.nn.modules.module.ModuleAttributeError: 'Inception3' object has no attribute 'features'