Compute true loss Flax examples
# What does this PR do?

'True' losses should be computed in Flax examples, as [discussed](https://github.com/huggingface/transformers/pull/18297#discussion_r931971230) with @sanchit-gandhi.


## Who can review?
cc @sanchit-gandhi @patrickvonplaten 


_The documentation is not available anymore as the PR was closed or merged._<|||||>@duongna21 super sorry it seems like the git commit history got messed up :-/ Any chance you could re-submit your PR? 