Pin transformers version for MII tests (#5629)
MII legacy tests use `from transformers import Conversation`
[here](https://github.com/microsoft/DeepSpeed-MII/blob/c171c4ee290e96c0d3e618b654be8add5eca973b/mii/legacy/method_table.py#L8).
Conversation was removed from transformers
[here](https://github.com/huggingface/transformers/pull/31165) so we pin
to a version before that before unpinning.
