In general we don’t consider what backward functions we use part of the public api, so unfortunately they aren’t super well documented. They are subject to change, so try not to rely on that behavior.
For example when you call max(tensor) in versions>=1.7, the grad_fn is now UnbindBackward instead of SelectBackward because max is a python builtin that relies onTensor.__iter__.
The behavior for torch.max to evenly distribute gradients is also new in 1.7 and that is documented in the release notes here [0].
[0] Release PyTorch 1.7 released w/ CUDA 11, New APIs for FFTs, Windows support for Distributed training and more · pytorch/pytorch · GitHub 7