'Return the Token for `access_token_key` or raise `InvalidTokenError`.
`request`: The Django request object.
`oauth_request`: The `oauth2.Request` object.
`consumer`: The Consumer that made the request.
`access_token_key`: The token key used to make the request.'

def get_access_token(self, request, oauth_request, consumer, access_token_key):

    raise NotImplementedError
