Match magnolify-tensorflow behavior w.r.t. nested types

[nevillelyh]

Magnolify flattens nested types as "field_a.field_b". Need to think about how we handle this here.

[nevillelyh]

More specifically one can write nested case class as Parquet file with parquet-types and might want to read them as TF Example which doesn't support nesting.

Alternatively we can ask users to always do case class -> Example using magnolify-tensorflow and then write flattened parquet-tensorflow instead.