Add score-based filtering and progress bar for predictions

[amrithasp02]

Included an option to filter the detections based on confidence scores. I added this as a step prior to NMS within the same function since it seemed easier & straightforward. But let me know if you think having it as a separate function is a better approach.

Also added the progress bar within the predict_as_generator method.

Note: Ignore changes made in detector.py. It has a previous commit that I reverted but I have changed the name of a variable. The logic hasn't changed.

[amrithasp02]

Now you can include --iou-threshold and --min-confidence as flags in the generate_predictions.py entrypoint script