Use py_func to yield elements from the given generator.

def make_input_fn_from_generator(gen):
  """Use py_func to yield elements from the given generator."""
  first_ex = six.next(gen)
  flattened = tf.contrib.framework.nest.flatten(first_ex)
  types = [t.dtype for t in flattened]
  shapes = [[None] * len(t.shape) for t in flattened]
  first_ex_list = [first_ex]

  def py_func():
    if first_ex_list:
      example = first_ex_list.pop()
    else:
      example = six.next(gen)
    return tf.contrib.framework.nest.flatten(example)

  def input_fn():
    flat_example = tf.py_func(py_func, [], types)
    _ = [t.set_shape(shape) for t, shape in zip(flat_example, shapes)]
    example = tf.contrib.framework.nest.pack_sequence_as(first_ex, flat_example)
    return example

  return input_fn