code: def get_avatar(self, subject_descriptor, size=None, format=None):
          
          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)
docs: GetAvatar.
          [Preview API]
          :param str subject_descriptor:
          :param str size:
          :param str format:
          :rtype: :class:`<Avatar> <azure.devops.v5_1.graph.models.Avatar>`