Since multi differentiation is not yet in stable, maybe the response is simply that something is not implemented yet.

Otherwise, why is the last line raising the error in comment?

```
from torch import nn, Tensor
from torch.autograd import Variable, grad
criterion = nn.MSELoss()
x = Variable(Tensor(5).normal_(), requires_grad = True)
y = Variable(Tensor(5).normal_())
l1 = (x - y).pow(2).sum().div(y.numel())
a = grad(l1, x, create_graph = True)
b = grad(a[0][3], x)
l2 = criterion(x, y)
a = grad(l2, x, create_graph = True)
b = grad(a[0][3], x) # "RuntimeError: differentiated input is unreachable"
```