Add nn::AdaptiveAvgPool2D

[quietlychris]

Re-opening per #215:

I was working on implementing AlexNet in dfdx per https://github.com/LaurentMazare/tch-rs/blob/main/src/vision/alexnet.rs, but have gotten a bit blocked by not having this layer type available. Would it be possible to add this, or alternatively to have some documentation on constructing the equivalent using GlobalAvgPool that was mentioned in that issue?