`y1`

is ok, but `y2`

lose the gradient of `a`

```
import torch
a = torch.tensor(0.0)
b = torch.tensor(0.0)
a.requires_grad = True
b.requires_grad = True
cosa = torch.cos(a)
cosb = torch.cos(b)
y1 = cosa + cosb
y2 = torch.tensor([
cosa, cosb
])
y2.sum().backward() # RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn
print("a has grad ", a.requires_grad)
print("a grad", a.grad)
```

Updated:

Besides, I need y2 to be matrix to support following operations.

```
y2 = torch.tensor([
cosa, cosb
])
mat = torch.tensor([
[1.0, 2.0],
[3.0, 4.0]
])
y2 = torch.matmul(y2, mat)
y2.sum().backward()
```