Copy a subset of hparams to target_hparams.

def update_hparams_from_hparams(target_hparams, source_hparams, prefix):
  """Copy a subset of hparams to target_hparams."""
  for (param_name, param_value) in six.iteritems(source_hparams.values()):
    if param_name.startswith(prefix):
      target_hparams.set_hparam(param_name[len(prefix):], param_value)