src/transformers/models/convbert/modeling_tf_convbert.py/TFConvBertMainLayer/get_head_mask
class TFConvBertMainLayer:    def get_head_mask(self, head_mask):
        if head_mask is not None:
            raise NotImplementedError
        else:
            head_mask = [None] * self.config.num_hidden_layers

        return head_mask