class_docstr: An attention layer that calculates attention scores using
  a dot product.
code: def score_fn(self, keys, query):
      return att_sum_dot(keys, query)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from __future__ import unicode_literals
  import abc
  import six
  import tensorflow
  from tensorflow.python.framework import function
  from seq2seq.graph_module import GraphModule
  from seq2seq.configurable import Configurable
label_desc: Use image node_id 1 for calling the AttentionLayerDot obj's underlying member method code with example usage: obj.score_fn(keys, query) and returns: att_sum_dot