Do we need to add `.detach()` after `var` in `INN.BatchNorm1d`?

[Zhangyanbo]

In INN.BatchNorm1d, the forward function is:

def forward(self, x, log_p=0, log_det_J=0):
        
        if self.compute_p:
            if not self.training:
                # if in self.eval()
                var = self.running_var # [dim]
            else:
                # if in training
                # TODO: Do we need to add .detach() after var?
                var = torch.var(x, dim=0, unbiased=False) # [dim]

            x = super(BatchNorm1d, self).forward(x)

            log_det = -0.5 * torch.log(var + self.eps)
            log_det = torch.sum(log_det, dim=-1)

            return x, log_p, log_det_J + log_det
        else:
            return super(BatchNorm1d, self).forward(x)

Do we need to requires var has gradient information? It seems not training BatchNorm1d, but training modules before it. Is there any references on this?

[Zhangyanbo]

Compare to nn.BatchNorm1d:

x = torch.randn((5, 3))
bn = nn.BatchNorm1d(3, affine=False)

bn(x)

The output is:

tensor([[-1.6941,  0.2933, -0.2451],
        [-0.1313, -0.2711,  1.4740],
        [ 0.2754, -0.2282,  0.4445],
        [ 0.1287, -1.4409, -0.0721],
        [ 1.4213,  1.6469, -1.6014]])

So, if we do not require affine in bn, we don't need gradient for BatchNorm.

[Zhangyanbo]

Experiments show that if we add .detach(), the training loss will not decrease. While if I added .detach(), it works. So, in the latest version, I added a parameter requires_grad:bool to INN.BatchNorm1d.