Fix weight decay masking in `run_flax_glue.py`
# What does this PR do?

Fixes #11936

In addition to the changes discussed in the issue, I combined `traverse` and `decay_path` into one function `decay_mask_fn` to simplify the implementation. 

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- [ ] Did you write any new necessary tests?


## Who can review?

@patrickvonplaten 
That's great - thanks a lot for the fix @n2cholas :-) I'll re-run the FlaxGlue suit today with your fix to check if results improve <|||||>Thanks a lot for the fix @n2cholas - I reran the eval train+eval script & updated the results accordingly