Suppose we have a tensor T with size(b, h * w, d) and an  tensor Index with size(b, h * w, k)-> with range(0, h*w-1) which stores the k different indexes of tensor T for each value of b * h * w.
How to generate indexed tensor from T with size (b, h * w, k, d) without extra memory cost ?
A memory cost way to implement this is:
T= T.unsqueeze(1).expand(b,h*w,h*w,d)
Index = I.unsqueeze(2).expand(b,h*w,k,d)
res =torch.gather(T,2,Index)