Fix Mask

AR/models/structs.py

@@ -78,6 +78,7 @@ class T2SSession:
                 pos = int(self.x_lens[bs].item())
                 mask = torch.zeros(pos + y_len, pos + y_len).bool()
                 mask[:, :pos].fill_(True)
-                mask[-y_len:, -y_len:] = ~torch.triu(torch.ones(y_len, y_len, dtype=torch.bool), diagonal=1)
+                if y_len > 0:
+                    mask[-y_len:, -y_len:] = ~torch.triu(torch.ones(y_len, y_len, dtype=torch.bool), diagonal=1)
                 attn_mask.append(mask)
             self.attn_mask_nested = torch.nested.nested_tensor(attn_mask)