[Flax test] Add require pytorch to flix flax test
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes Flaky CI.

Currently the flax tests are failing which IMO is because of a missing `require_torch` in the flax test. 
This PR should fix it -> @mfuntowicz could you take a look? 


## Before submitting
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