Hello. Is there a simple way to sum 2D tensors’ values with torch.scatter_add() by starting from the adjacency matrix? This is an example explaining what I would like to obtain:

```
x = torch.tensor([[1., 2.],
[3., 4.],
[5., 6.],
[7., 8.]])
adj = torch.tensor([[1, 1, 0, 1],
[1, 1, 1, 0],
[0, 1, 1, 0],
[1, 0, 0, 1]])
```

The output tensor after the operations should be

```
[[11., 14.],
[9., 12.],
[8., 10.],
[8., 10.]]
```

Meaning that sums should be computed in this way:

```
[ [1+3+7, 2+4+8],
[3+1+5, 6+2+4],
[5+3, 6+4],
[7+1, 8+2]]
```

I remark that solving this with scatter_add() is quite important (but, of course, I am open to other solutions!).