Return a `measurements.Dimension` instance.

def _maybe_make_dimension(self, dimension):
    """Return a `measurements.Dimension` instance."""
    # For backwards compatibility the argument can be either a Dimension, a
    # string or a `units.UnitDescriptor`.
    if isinstance(dimension, Dimension):
      return dimension
    if isinstance(dimension, units.UnitDescriptor):
      return Dimension.from_unit_descriptor(dimension)
    if isinstance(dimension, str):
      return Dimension.from_string(dimension)

    raise TypeError('Cannot convert %s to a dimension', dimension)