Code optimization for dropout operation

[hi-zhengcheng]

Thanks for your nice work. I just read through the code and find a small problem in dropout operation:

https://github.com/hkust-vgd/shellnet/blob/fe5cf5f3806e618c23e161b0fb8a57387268df78/shellconv.py#L119

Actually this dropout operation has no effect if you don't pass training=is_training to it. The default value for training parameter is False so it will return the input untouched.

But it's ok, it doesn’t influence the experiment. Maybe you can fix it in your spare time, or just remove dropout to make code clearer.