Fix data shape for classification images (#9)

Data argument (https://www.tensorflow.org/api_docs/python/tf/summary/image#arguments) for `tf.summary.image` expects data to have the shape `[k, h, w, c]`

training/callbacks/classification_images.py

@@ -121,4 +121,4 @@ def on_epoch_end(self, epoch, logs=None):
         # Sort by hash so the images show up in the same order every time
         with self.writer.as_default():
             for i, img in enumerate(sorted(images, key=lambda image: image[0])):
-                tf.summary.image("images/{}".format(i), tf.expand_dims(img[1], axis=-1), step=epoch, max_outputs=1)
+                tf.summary.image("images/{}".format(i), tf.expand_dims(img[1], axis=0), step=epoch, max_outputs=1)