guide: Add checkpoints example with downstream stages

[daavoo]

All of the current examples use a pipeline where the stage using checkpoints is the last one. I think it would be nice to have an example where there are some downstream stages depending on the stage using checkpoints.

I think this kind of pipeline is more realistic as users would probably want to do something with the model after training, like exporting to other formats for inference, running inference on other subsets of the data, deploying the model, etc.

We should explain how to use checkpoints in this scenario given that the downstream stages usually expect to have access to the best checkpoint, which is not necessarily the latest as we assume in the workflows currently documented.