fix: ScaleModule and SumModule for DiagHessian.

[hlzl]

Partially fixes #316 . ScaleModule is also used for torch.nn.Identity.

Not sure if hessian_is_zero() should always return True for those two modules.
Same with accumulate_backpropagated_quantities() which concats dicts instead of tensors as for the DiagGGN.

[hlzl]

Commit 74c4173 allows to compute the Hessian diagonal even if there is a batch norm in the network by simply not computing the Hessian elements for the batch norm layer.

Not sure if this is a reasonable approach, however, this can be used as a quick fix.

The other diagonal elements can then be extracted as following:

hessian_diag_wo_bn = torch.cat(
    [
        p.diag_h_batch.view(batch.shape[0], -1)
        for p in model.parameters()
        if "diag_h_batch" in p.__dict__.keys()
    ],
    dim=1,
)

[hlzl]

Commit 48f03e9 tries to actually compute the diagonal elements of the Hessian for the batch norm layer.

If one of you could have a quick look at the commits to see if they make any sense, would really appreciate it. @f-dangel @fKunstner

Thank you!

[f-dangel]

Hi,

just wanted to let you know I read your message above. Please don't expect any reaction before the ICLR deadline (Sep 28)