Support additional data in DriverMetadata

[mrossinek]

What is the expected enhancement?

#573 adds a workaround for the DriverMetadata object when passed through a transformer.

A useful enhancement would be to support additional data in this container which can store transformer-related information. This would serve the purpose of providing full information of how a certain driver result was obtained aiding in future reproducability.

A very crude idea would be to add string representations of the transformer instances which were applied to a given driver result. Then DriverMetadata could simply store a list of the applied transformers which a user could read and interpret in order to reproduce a certain result.

Here are some more specific implementation ideas:

• implement BaseTransformer.__repr__(self) for all its subclasses
• add a DriverMetadata.transformers: List[str] field and increment it's VERSION class attribute (if this get's done after Qiskit Nature 0.4 is released)
• append the transformers repr(self) representation to the DriverMetadata.transformers list when transforming this object (see fix: catch DriverMetadata-related warnings during transform #573 for the code location where this would need to be added)
• handle DriverMetadata.transformers during to_hdf5 and from_hdf5 of the DriverMetadata class (versioning needs to be handled if the VERSION had to be increased)
• unittest all of the above

[mrossinek]

I had this issue in my inbox again because somebody left a comment here which now appears to have been deleted.
Nonetheless, I wanted to leave a short update here, stating that the DriverMetadata is likely to undergo a significant re-design as part of #701 and #705. Thus, I will mark this issue as requiring more information and highly recommend anyone who would want to start on this, to first check in on the current status of its design 👍