Add generic BaseConverter class. (#1024)
Add generic BaseConverter class. (#1024)

* Add BaseConverter class.

* Add str_to_item_fn accessor.

* lint

* lint

* lint

* lint

* lint

* Fix typo.

* Fix maybe_sample

jesseengel/magenta,adarob/magenta,jesseengel/magenta,magenta/magenta,magenta/magenta,adarob/magenta