Created by: tonyyang-svail

A variable should be able to access its gradient. Ideally, the access should be a smart pointer.

Question: should a variable hold

1. a shared_ptr to its gradient?
2. a weak_ptr to its gradient?

Case 0: forward

while (true) {
  reset_global_tape();
  loss = model.Forward(data);
}

Case 1: forward, backward

while (true) {
  reset_global_tape();
  loss = model.Forward(data);
  loss.Backward();
}

Case 2: forward, backward, optimize

while (true) {
  reset_global_tape();
  loss = model.Forward(data);
  loss.Backward();
  sgd(model.Params());
}

Case 3: Release Memory on Backward

We can release memory aggressively. During backward, we can delete the OpHandle once we have finished its backward. Since all the variable is managed by smart pointers, the memory is automatically released when its ref_count goes to 0.

In the current implementation, shared_ptr can't solve case 1, while weak_prt can't solve case 3.