Supported models

BackPACK expects models to be sequences of PyTorch NN modules. For example,

model = torch.nn.Sequential(
        torch.nn.Linear(784, 64),
        torch.nn.ReLU(),
        torch.nn.Linear(64, 10)
)

This page lists the layers currently supported by BackPACK.

Do not rewrite the forward() function of the Sequential or the inner modules! If the forward is not standard, the additional backward pass to compute second-order quantities will not match the actual function. First-order extensions that extract information might work outside of this framework, but it is not tested.

---

For first-order extensions

BackPACK can extract more information about the gradient with respect to the parameters of the following layers;

• torch.nn.Linear
• torch.nn.Conv1d, torch.nn.Conv2d, torch.nn.Conv3d
• torch.nn.ConvTranspose1d, torch.nn.ConvTranspose2d, torch.nn.ConvTranspose3d

First-order extensions should support any module as long as they do not have parameters, but some layers lead to the concept of “individual gradient for a sample in a minibatch” to be ill-defined, as they introduce dependencies across examples (like torch.nn.BatchNorm).

---

For second-order extensions

BackPACK needs to know how to propagate second-order information. This is implemented for:

Parametrized layers	torch.nn.Conv2d
	torch.nn.Linear
Loss functions	torch.nn.MSELoss
	torch.nn.CrossEntropyLoss
Layers without parameters	torch.nn.MaxPool2d torch.nn.AvgPool2d
	torch.nn.Dropout
	torch.nn.ReLU torch.nn.Sigmoid torch.nn.Tanh

The other convolution layers (Conv1d, Conv3d, and ConvTransposeNd) are not yet supported.