OPT: Fix batched generation with FLAX
# What does this PR do?

Fixes #20666 

OPT numerical masking was using `-inf` where the attention mask was `0`, which in turn caused the hidden states to be `nan` and derail the whole generation. Changing to a common masking value (`-1e9`) fixes the issue. I've also taken the opportunity to re-enable the commented out tests :)
_The documentation is not available anymore as the PR was closed or merged._