Clarification on `torch.autograd.grad`'s `only_inputs` boolean?

I have a few questions about what only_inputs for torch.autograd.grad really does. When True does this mean that gradients are computed for no other leaves? Or that no gradient flows past the inputs. Similarly, does False mean that gradient will flow beyond the inputs to leaves further in the network? Or does False mean it will flow to everything between inputs and outputs, but not past inputs?

In particular, what happens when using allow_unused=True where all inputs are “unused”? Can I pass require_grad=False variables as “unused” as input, then use grad to compute gradients on leaf nodes based on an output dependent on the unused inputs?

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Also interested in this, can anyone help?