[`SwitchTransformers`] Fix return values
# What does this PR do?
 
The previous version of the code would always return `None` values for the router losses, since the `if output_router_probs` was wrapper around `if labels is not None`. But router logits can still be computed wthout  labels. 
Also returns tensors instead of None, this follows our usual API, and is less prone to errors. 
_The documentation is not available anymore as the PR was closed or merged._