I want to understand how does ReLU backward, I find it is implemented by torch._C._nn.threshold, not autograd.Function object.
I think all object from torch._C._nn is defined by aten/src/Aten/nn.yaml, and a backward implementation of torch._C._nn.threshold is THNN_(Threshold_updateGradInput), however, I finds it accept THTensor *input, I do not know where does it get this tensor while it is not an autograd.Function which can save input as ctx.