Error when parsing graph_def from string

The problem here is that you are trying to parse a SavedModel protocol buffer as if it were a GraphDef. Although a SavedModel contains GraphDef, they have different binary formats. The following code, using tf.saved_model.loader.load() should work:

import tensorflow as tf

with tf.Session(graph=tf.Graph()) as sess:
    tf.saved_model.loader.load(
        sess, [tf.saved_model.tag_constants.SERVING], "models/TEST-3")