Initializer for _operation_name_to_id.

    Returns:
      a {string: int}, mapping operation names to their index in _operations.

def _initialize_operation_name_to_id(self):
    """Initializer for _operation_name_to_id.

    Returns:
      a {string: int}, mapping operation names to their index in _operations.
    """
    operation_name_to_id = {}
    for i, operation in enumerate(self._operations):
      operation_name_to_id[operation.name] = i
    return operation_name_to_id