TextualGradientDescentwithMomentum missing return statement

[jeffery1236]

In the _update_prompt method of TextualGradientDescentwithMomentum, I found a missing return statement.

https://github.com/zou-group/textgrad/blob/ee13ed76c57685d168453012bc9b8333df5c0547/textgrad/optimizer/optimizer.py#L233

[mertyg]

ah, great catch. we never used this after all, but we should fix this in the next release. thank you @jeffery1236 !