docs: add explicit source (matching PyTorch guide) (#12844)
## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
The PyTorch guide page has this, but it's missing from this example
(which is otherwise identical to the PyTorch guide page). I think it
would be helpful to include it here too.
## Test Plan
Docs.
<!-- How was it tested? -->
Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
