Stack multiple Tensors to make a new dimension.

  Args:
    xs: a list of Tensors with identical shapes.
    dim_name: a string (name of the new dimension)
    axis: an integer (index of the new dimension in the output shape)
    name: an optional string

  Returns:
    a Tensor

def stack(xs, dim_name, axis=0, name=None):
  """Stack multiple Tensors to make a new dimension.

  Args:
    xs: a list of Tensors with identical shapes.
    dim_name: a string (name of the new dimension)
    axis: an integer (index of the new dimension in the output shape)
    name: an optional string

  Returns:
    a Tensor
  """
  ret = StackOperation(xs, dim_name, axis, name).outputs[0]
  return ret