added Builder.copy()

[zilto]

This is useful in the Jupyter Magic V2, plugins and other scenarios where we want users to be able to pass a Builder() and another function be responsible for completing the Driver creation.

Changes

• add Builder().copy() as currently implemented in the Jupyter magics V2 PR

How I tested this

• check the copy points to a different object
• check the copy and original have equal attributes

Notes

Currently, the copied Builder() holds references to the original Builder attributes. AFAIK, this means the original Builder() can be garbage collected, but not its attribute (assuming they're useful to the new Builder copy. An odd edge case would be:

1. create Builder foo with stateful adapters
2. copy Builder foo to create Builder bar
3. create Driver foo and Driver bar from both.
4. both Driver seemingly share an adapter with the same state, which could be odd for CachingAdapter, ExperimentTracker, and others given they store metadata as they progress through the DAG