FLAN-T5-small model run sentiments.ipynb demo gives so so many ""using bos_token ,but it is not set yet" lines"

### 🐛 Describe the bug

when I try to replace GPT2 with FLAN-T5-small model to run this demo:

https://colab.research.google.com/github/CarperAI/trlx/blob/main/examples/notebooks/trlx_sentiments.ipynb

it output a lot of "using bos_token ,but it is not set yet" lines

and I find when I set the samples smaller, like smaples = imdb['text'][:20]
the lines number is just 20
so ..when I use the whole imdb dataset ,the output will be 50k lines.
And before its final 50k-output done ,nothing else will be seen.

### Which trlX version are you using?

latest at 20230416

### Additional system and package information

colab Python env py39