Hi,

What is the possible way to perform the gradient on the index?

I am trying to do something like “spatial transform network”.

However, I need to do the sample on a branch of input so I want to try this by my own.

something like:

```
# build the index from range
x,y,u,v = meshgrid(...)
# select the pixel from indexs
torch.gather(img,4,torch.stack([x,y,u,v],-1))
```

This will done by tensorflow as:

```
tf.gather_n(img,torch.stack([x,y,u,v],-1))
```

however, I get the error on pytorch that `Gather can't differentiate the index`

any ideas about this problem?

thanks in advanced.