Encoder APIs: Features

[FrancescoSaverioZuppichini]

Would be nice to also pass the stages + features_widths parameters to a Encoder so we can on-the-fly change which layer is used.

ResNetEncoder(32, 64, stages=lambda enc: [enc.layers[0]], enc.layers[1]] , features_widths=lambda enc: [enc.start_features, enc.widths[0])

With the current API we could subclass the encoder

MyResNetEncoder(ResNetDecoder):

    @property
    def stages():
        return [self.stem[0],
                self.layers[1]]

    @property
    def features_widths():
        return [self.start_features[0],
                self.widths[0]]