Add utility for debugging intermediate graph nodes values

Similar to pytorch's [register_hook](https://docs.pytorch.org/docs/stable/generated/torch.Tensor.register_hook.html).

A POC-grade `register_hook` which I've used successfully can be found [here](https://github.com/soundradix/autograd/commit/dd8566b8c5ad33c6cfca12067484a237a5616a72).