same results as traditional dropout if block_size=1

[sergulaydore]

Thanks for the code!
I was wondering if you get the same results as traditional dropout when block_size=1.
Based on my experiments, using F.dropout2d for traditional dropout, I cannot confirm this.

[miguelvr]

right now the behaviour should be very similar to dropout, but it has some subtle changes.

Particularly, at the moment the mask is broadcasted to all the channels, which dropout doesn't do.

Once issue #24 is fixed, behaviour should be similar.