Dreambooth: reduce VRAM usage (#2039)
* Dreambooth: use `optimizer.zero_grad(set_to_none=True)` to reduce VRAM usage
* Allow the user to control `optimizer.zero_grad(set_to_none=True)` with --set_grads_to_none
* Update Dreambooth readme
* Fix link in readme
* Fix header size in readme
