I need to perform some basic numerical integrations in pytorch and am currently investigating feasible methods. As a validity check for using the new `torch.trapz`

function, I implemented the Matlab example on multiple numerical integration:

```
import torch
dx = 0.1
x = torch.arange(-3, 3, dx)
y = torch.arange(-5, 5, dx)
X, Y = torch.meshgrid(x, y)
F = X**2 + Y**2
result = torch.trapz(torch.trapz(F, dx=dx, dim=0), dx=dx, dim=0)
# Returns tensor(646.9882)
```

The result is quite far away from what Matlab produces (680.200) and the exact value of 680. Is this expected? If yes, why so?

What would be a simple alternative to using `torch.trapz`

?