Add support for loading flattened meta_tags.csv
🚀 Feature
Motivation
PL+TensorBoard can log hierarchical dict after #1152, however, meta_tags.csv has been disabled by the change.
Pitch

Make meta_tags.csv back to a hierarchical dict based on their delimiter.

Alternatives
62de794
Additional context

We can consider the deprecation of meta_tags.csv, then adopt config.yaml.
We can interpret primitive-type parameters through the files, so it is needed to rethink parameter sanitization or update docs.