'Collect a hash of the course\'s grading policy, storing it as a
`transformer_block_field` associated with the `GradesTransformer`.'

@classmethod
def _collect_grading_policy_hash(cls, block_structure):

    course_location = block_structure.root_block_usage_key
    course_block = block_structure.get_xblock(course_location)
    block_structure.set_transformer_block_field(course_block.location, cls, 'grading_policy_hash', cls.grading_policy_hash(course_block))
