Torch-Scripting the Swish function from the PyTorch Video implementation of X3D

I am trying to get a torchscript .pt file from a trained X3D network (a 3D CNN). One module
is causing the problem, in particular an implementation of the Swish activation given by:

import torch
import torch.nn as nn

class Swish(nn.Module):
    Wrapper for the Swish activation function.

    def forward(self, x):
        return SwishFunction.apply(x)

class SwishFunction(torch.autograd.Function):
    def forward(ctx, x):
        result = x * torch.sigmoid(x)
        return result

    def backward(ctx, grad_output):
        x = ctx.saved_variables[0]
        sigmoid_x = torch.sigmoid(x)
        return grad_output * (sigmoid_x * (1 + x * (1 - sigmoid_x)))

When I try to script the X3D network, I get the error specific to this module, which has a custom back-pass implementation. The error reads:

Could not export Python function call 'SwishFunction'. Remove calls to Python functions before export. Did you forget to add @script or @script_method annotation? If this is a nn.ModuleList, add it to __constants__:

Any thoughts how to fix this?