Fixed bugs in generate_dataset.py

[yaoxiao6]

Fixed bugs:

1. The latest version of pytube can work now. We just have to reinstall it. I added a comment to remind others of this approach.
2. videoname is referenced before the declaration. I declared this variable and checked None before using it to solve the error.
3. At this line, we should use [-1], not [-2]: if videoname_candinate.split('.')[-1] == '/current_' + mode:. Because ./current_train is the part we want to find.