message: Show an error message in _einsum_reduction ValueError.
  If `tf.einsum()` is called and the length of the axis labels do not match the
  rank of the tensor, the ValueError now states what the problem is, which tensor
  is causing it, and what the rank and length of the labels are.
  RELNOTES: n/a
diff: @@ -251,9 +251,13 @@ def _einsum_reduction(t0, t0_axis_labels, t1, t1_axis_labels, axes_to_sum):
  `t1_axis_labels`.
  """
  if len(t0_axis_labels) != len(t0.get_shape()):
  - raise ValueError()
  + raise ValueError(
  + 'Tensor t0 of rank %d does not match einsum reduction of length %d' %
  + (len(t0.get_shape()), len(t0_axis_labels)))
  if len(t1_axis_labels) != len(t1.get_shape()):
  - raise ValueError()
  + raise ValueError(
  + 'Tensor t1 of rank %d does not match einsum reduction of length %d' %
  + (len(t1.get_shape()), len(t1_axis_labels)))
  # This function computes the result of a two-argument einsum() using batch
  # matrix multiplication. This involves
  