log_artifact: add cache option, only write to dvc.yaml if metadata ex…

[dberenbaum]

Extracted from #613.

• Artifacts will only be annotated in dvc.yaml:artifacts if some metadata is provided (type, name, desc, labels, or meta). This is a breaking change, but I can't see how anyone is making use of this without any metadata since it won't be used by the model registry. It's needed in auto save model in lightning #613 so we can cache a whole directory of checkpoints without annotating the whole directory.
• Added cache kwarg to log_artifact (defaults to True) so that it's possible to annotate the artifact metadata without caching the object. It's needed in auto save model in lightning #613 so we can annotate the best artifact in dvc.yaml without re-caching (since it overlaps with the cached checkpoints directory).

There are probably workarounds for both in #613, but I think this flexibility to choose when to cache and when to annotate in dvc.yaml is useful. It also should close #572 and #551 (edit: and partially address #551) since it decouples caching and annotation.

[dberenbaum]

Looks like there are unrelated errors coming from the keras callback.

[daavoo]

Agree it closes #572
But I don't think it closes the external part of #551