[FlaxGPTJ] Fix bug in rotary embeddings
# What does this PR do?

The rotary embeddings in `FlaxGPTJ` are computed in-correctly and they don't match with the PT version. This PR fixes  `rotate_every_two` function to match with PT version.

Thanks a lot @ydshieh for finding this issue!

Fixes #16288
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Shouldn't the slow test have failed here?

The difference(s) - for hidden states - I got are in the range like of `1e-2`. Not sure if this will affect the outputs of slow test.
I can run the slow tests to see what happen internally.<|||||>> > Shouldn't the slow test have failed here?
> 
> The difference(s) - for hidden states - I got are in the range like of `1e-2`. Not sure if this will affect the outputs of slow test. I can run the slow tests to see what happen internally.

Yes, the diff for the output was surprisingly small and didn't affect generation when I last tested.