footprint, and can modestly improve performance. However, it
         changes certain behaviors. For example: 1. When the user
         tries to access a gradient and perform manual ops on it, a
         None attribute or a Tensor full of 0s will behave
         differently. 2. If the user requests
         "zero_grad(set_to_none=True)" followed by a backward pass,
         ".grad"s are guaranteed to be None for params that did not
         receive a gradient. 3. "torch.optim" optimizers have a
         different behavior if the gradient is 0 or None (in one case
         it does the step with a gradient of 0 and in the other it
         skips the step altogether).