Efficient Conv Ops were removed accidentally

[ricardoV94]

Description

We removed the deprecated nnet module in #111

But we wanted to keep the Convolution Ops. Unfortunately we keept only the "abstract" ones, and not the fast variations implemented in conv.py. We should bring them back, including the rewrites that replace abstract forms by the equivalent fast ones.

https://discourse.pymc.io/t/very-slow-2d-convolution-with-pytensor/12118/6

[jessegrabowski]

Also related to #275 I think