Transformer step (with or without 'custom') fails

[paprocki-r]

          Thank you so much for reporting this issue. We recently merged a fix in [mlflow master](https://github.com/mlflow/mlflow/pull/7626) to resolve this issue. 

Let me know if that fixes the issue for you.

Originally posted by @sunishsheth2009 in #29 (comment)

So the error still occurs

[sunishsheth2009]

The PR above was merged on December 29th and the latest release was released on 25th December. We do plan to do another release soon but until then can we try installing MLflow from master and see if that solves the problem? Thank you :)