import torch
import torchvision
from torchvision.models.detection.faster_rcnn import FastRCNNPredictor

def get_detection_model(num_classes):
    """
    Original helper to instantiate PyTorch Faster R-CNN with ResNet50 backbone.
    """
    # Load a model pre-trained on COCO
    model = torchvision.models.detection.fasterrcnn_resnet50_fpn(pretrained=True)

    # Get the number of input features for the classifier
    in_features = model.roi_heads.box_predictor.cls_score.in_features
    # Replace the pre-trained head with a new one
    model.roi_heads.box_predictor = FastRCNNPredictor(in_features, num_classes)
    
    return model