Discards alpha channel (if any) in tfci.py.

PiperOrigin-RevId: 362350339
Change-Id: Icddf9fe099c9fccfa7689c93dd68f797a5049539

Author: Johannes Ballé

models/tfci.py

@@ -39,7 +39,7 @@
 def read_png(filename):
   """Loads a PNG image file."""
   string = tf.io.read_file(filename)
-  image = tf.image.decode_image(string)
+  image = tf.image.decode_image(string, channels=3)
   return tf.expand_dims(image, 0)