'Ensure client is authorized to use the grant_type requested.
:param client_id: Unicode client identifier
:param grant_type: Unicode grant type, i.e. authorization_code, password.
:param client: Client object set by you, see authenticate_client.
:param request: The HTTP Request (oauthlib.common.Request)
:rtype: True or False
Method is used by:
- Authorization Code Grant
- Resource Owner Password Credentials Grant
- Client Credentials Grant
- Refresh Token Grant'

def validate_grant_type(self, client_id, grant_type, client, request, *args, **kwargs):

    raise NotImplementedError(u'Subclasses    must    implement    this    method.')
