text: Title: KeyError: 'resnext50_32x4d'
  Body: what's wrong with resnext50_32x4d?
  `model = smp.Unet("resnext50_32x4d", encoder_weights="imagenet", classes=4, activation='sigmoid')`
  error:
  KeyError: 'resnext50_32x4d' 
  However, it's defined here:
  https://github.com/qubvel/segmentation_models.pytorch/blob/master/segmentation_models_pytorch/encoders/resnet.py#L85