Pass a dummy input through the model `m` to get the various sizes of activations.

def model_sizes(m:nn.Module, size:tuple=(64,64))->Tuple[Sizes,Tensor,Hooks]:
    "Pass a dummy input through the model `m` to get the various sizes of activations."
    with hook_outputs(m) as hooks:
        x = dummy_eval(m, size)
        return [o.stored.shape for o in hooks]