Suppose A is a 2D tensor with shape (3,4). B is a 2D tensor with shape (k,2) specify k selected index. The output is a tensor C with shape (k) described as:

```
C = []
for i in range(len(B)):
C.append(A[B[i][0]][B[i][1]])
C = torch.tensor(C)
```

I wonder if there exists some Pytorch API equivalent to the above code segment?