def parse_dtype_info(flags):
  """Convert dtype string to tf dtype, and set loss_scale default as needed.

  Args:
    flags: namespace object returned by arg parser.

  Raises:
    ValueError: If an invalid dtype is provided.
  """
  if flags.dtype in (i[0] for i in DTYPE_MAP.values()):
    return  # Make function idempotent

  try:
    flags.dtype, default_loss_scale = DTYPE_MAP[flags.dtype]
  except KeyError:
    raise ValueError("Invalid dtype: {}".format(flags.dtype))

  flags.loss_scale = flags.loss_scale or default_loss_scale