TF: Add missing cast to GPT-J
# What does this PR do?

Adds a missing cast, which was breaking the tests for mixed precision (which, for some weird cause, was also causing subsequent tests to fail 🤔 ) 

All slow tests pass after this change.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Ah, that explains why following tests fail! Changing it 👍 
