torch.func.stack_module_state

torch.func.stack_module_state(models) → params, buffers

Prepares a list of torch.nn.Modules for ensembling with vmap().

Given a list of M nn.Modules of the same class, returns two dictionaries that stack all of their parameters and buffers together, indexed by name. The stacked parameters are optimizable (i.e. they are new leaf nodes in the autograd history that are unrelated to the original parameters and can be passed directly to an optimizer).

Here’s an example of how to ensemble over a very simple model:

num_models = 5
batch_size = 64
in_features, out_features = 3, 3
models = [torch.nn.Linear(in_features, out_features) for i in range(num_models)]
data = torch.randn(batch_size, 3)

def wrapper(params, buffers, data):
    return torch.func.functional_call(model[0], (params, buffers), data)

params, buffers = stack_module_state(models)
output = vmap(wrapper, (0, 0, None))(params, buffers, data)

assert output.shape == (num_models, batch_size, out_features)

When there’s submodules, this follows state dict naming conventions

import torch.nn as nn
class Foo(nn.Module):
    def __init__(self, in_features, out_features):
        super().__init__()
        hidden = 4
        self.l1 = nn.Linear(in_features, hidden)
        self.l2 = nn.Linear(hidden, out_features)

    def forward(self, x):
        return self.l2(self.l1(x))

num_models = 5
in_features, out_features = 3, 3
models = [Foo(in_features, out_features) for i in range(num_models)]
params, buffers = stack_module_state(models)
print(list(params.keys()))  # "l1.weight", "l1.bias", "l2.weight", "l2.bias"

Warning

All of the modules being stacked together must be the same (except for the values of their parameters/buffers). For example, they should be in the same mode (training vs eval).

Return type

Tuple[Dict[str, Any], Dict[str, Any]]