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_DecodeGif.pbtxt

@@ -21,6 +21,6 @@ uncompressed by running:
     convert $src.gif -coalesce $dst.gif
 
 This op also supports decoding JPEGs and PNGs, though it is cleaner to use
-`tf.image.decode_image`.
+`tf.io.decode_image`.
 END
 }