Added the FastAI UNet implementation

[ceferisbarov]

Following this discussion, I closed that PR and opened this one where I have added the FastAI implementation of UNet.

The model itself seems to have some problems (they exist in FastAI package, too). I will try to clarify and solve them.

[CarloLucibello]

A slightly modified UNet implementation can also be found in https://github.com/FluxML/model-zoo/pull/349

[darsnack]

Closing as stale