[Time Series Transformer] Add doc tests
# What does this PR do?

This PR improves the code snippets in the docs of Time Series Transformer and makes sure they are tested.
_The documentation is not available anymore as the PR was closed or merged._<|||||>LGTM! thanks!<|||||>@ydshieh always better to learn more than earn more<|||||>@NielsRogge for generation you can also use the other test batch if you like