GetAvatar.
        [Preview API]
        :param str subject_descriptor:
        :param str size:
        :param str format:
        :rtype: :class:`<Avatar> <azure.devops.v5_1.graph.models.Avatar>`

def get_avatar(self, subject_descriptor, size=None, format=None):
        """GetAvatar.
        [Preview API]
        :param str subject_descriptor:
        :param str size:
        :param str format:
        :rtype: :class:`<Avatar> <azure.devops.v5_1.graph.models.Avatar>`
        """
        route_values = {}
        if subject_descriptor is not None:
            route_values['subjectDescriptor'] = self._serialize.url('subject_descriptor', subject_descriptor, 'str')
        query_parameters = {}
        if size is not None:
            query_parameters['size'] = self._serialize.query('size', size, 'str')
        if format is not None:
            query_parameters['format'] = self._serialize.query('format', format, 'str')
        response = self._send(http_method='GET',
                              location_id='801eaf9c-0585-4be8-9cdb-b0efa074de91',
                              version='5.1-preview.1',
                              route_values=route_values,
                              query_parameters=query_parameters)
        return self._deserialize('Avatar', response)