Channels in mri_silhouettes

The model expects 3 channels but you provide 1 channel.

```
input = tf.keras.Input(
    shape=(256, 237, 1),
    name="input_mdrk_silhouette_continuous",
)
```

https://github.com/broadinstitute/ml4h/blob/master/model_zoo/silhouette_mri/train_models.py#L261

How do you account for the other 2 channels? @mklarqvist 