def forward(self, attn, normalize=True):
        """mask attention with the right span"""
        # batch and head dimensions are merged together, so separate them first
        self.clamp_param()
        if self._adapt_span_layer:
            attn = self._mask(attn)
        else:
            B = attn.size(0)  # batch size
            M = attn.size(1)  # block size
            attn = attn.reshape(B // self._n_head, self._n_head, M, -1)
            attn = self._mask(attn)
            attn = attn.view(B, M, -1)
        return attn
mask attention with the right span
understand_icl/fairseq/examples/adaptive_span/adaptive_span_attention.py