Implement convolutional layers and associated helpers

[jessegrabowski]

Convolutional layers remain important for computer vision problems even after the transformer revolution, so they are important to have:

• Conv2d
• ConvTranspose2d

For these, we need to address pymc-devs/pytensor#548

In addition, convolutional network designs typically include some kind of pooling layers:

• MaxPool2d
• AvgPool2d
• MaxUnpool2d

As well as helpers for padding images to the correct size:

• ReflectionPad2d
• ReplicationPad2d
• ZeroPad2d
• ConstantPad2d

A pt.pad function would be nice for implementing these; I opened an issue for that here pymc-devs/pytensor#743