tf.image.decode_image to tf.io.decode_image

tf.image.decode_image to tf.io.decode_image for API migration and consistency.
Please see https://www.tensorflow.org/api_docs/python/tf/io/decode_image

tensorflow/core/api_def/base_api/api_def_DecodePng.pbtxt

@@ -34,6 +34,6 @@ If needed, the PNG-encoded image is transformed to match the requested number
 of color channels.
 
 This op also supports decoding JPEGs and non-animated GIFs since the interface
-is the same, though it is cleaner to use `tf.image.decode_image`.
+is the same, though it is cleaner to use `tf.io.decode_image`.
 END
 }