Fixed minor changes, that resulted in code crash during runtime

Created by: yashYRS

Faced the following errors, while running the files. After making these minor changes, the code runs smoothly

1. Removed all instances of plt.hold(true) since the same has been deprecated. Source : https://stackoverflow.com/questions/53689386/python-3-pyplot-hold-is-deprecated

2. cv2.find_contours() : return type expected values are 3, or 2 depending on the version of cv2 installed. Changed code to handle all versions. Source : https://github.com/facebookresearch/maskrcnn-benchmark/issues/339