Get a UserWarning: Implicit dimension choice for softmax when training mini-RTS following Install tutorial.

[caidwang]

I get a User warning when start the training demo sh ./train_minirts.sh in the environment with python 3.6 and pytorch-cpu 0.3.1. It looks like this:

The warning seems to be not serious but I think it's a little confusing which dimension the softmax is working on (I guess it might be dim=0 ?). Whatever, I just suggest that this warning can be cleaned up by specifying the dimension exacter.