Pytorch: Matrix multiplication operator

[bartvm]

Instead of overloading the multiplication operator to do both element-wise and matrix-multiplication it would be nicer and much safer to just support Python's matrix multiplication operator (see PEP 465, A @ B is the matrix product, A * B the element-wise product).

[alexbw]

A good reason to use Python 3

[apaszke]

I had no idea there's a standardized symbol for matrix product. I think it's worth adding. Thanks!

[soumith]

this is now fixed in master