Making Conv2D unbiased by default, and adding Bias2D module

[coreylowman]

Resolves #100

Motivation: For most vision based networks these days, convolution layers are followed by batchnorm2d. Best practice is to always set bias=False in pytorch conv layers, since batchnorm2d will effectively nullify the bias with the normalization and also contains a bias of its own.

This PR removes bias from Conv2D, meaning dfdx is moving to you have to explicitly specify that its a biased conv by adding a Bias2D layer following:

type BiasedConv = (Conv2D<3, 5, 3>, Bias2D<5>);

Another change introduced because of this is that biases are now initialized to zero always, instead of the random uniform they were before. This is what tensorflow does with biases, so its still a standard initialization