Feature request: support placeholders in the data parameter of TrainingStep constructor

[lucaboulard]

Currently the data parameter to the TrainingStep constructor does not support placeholders, requiring the s3 location of the dataset to be known at workflow creation time.
It would be useful for this parameter to accept dynamic s3 uri, via placeholders.

[shivlaks]

Implemented in #142 which has now been merged 🎉