and I forgot about accessors. this is from docs:
torch::Tensor foo = torch::rand({12, 12});

// assert foo is 2-dimensional and holds floats.
auto foo_a = foo.accessor<float,2>();
float trace = 0;

for(int i = 0; i < foo_a.size(0); i++) {
  // use the accessor foo_a to get tensor data.
  trace += foo_a[i][i];
}