Generate: Export TF generate with a TF tokenizer
# What does this PR do?

See #22254 

As the title says, this PR adds the possibility to export TF generate with a TF-native tokenizer -- the full thing in a single TF graph 🤯 

The missing piece was removing a redundant `if` before `tf.while_cond` -- `tf.while_cond` checks the condition before running the body, so the existing `if` before it was redundant. It was also the root cause behind the error in #22254, so removing it was a double win 🎉 

A test was added to ensure we don't regress.
@Rocketknight1 no need to review, but FYI -- we can now compile the whole thing into a graph :D<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>This is amazing!