Make one hot encoder serialisable

[uri-granta]

Related issue(s)/PRs:

Summary

Quick fix to allow one hot encoders to be serialised. Further investigation will happen as part of #304.

(Note that second change to tf.debugging in encoder doesn't seem to be necessary for the serialisation to work, but is included for consistency.)

Fully backwards compatible: yes / no

PR checklist

• The quality checks are all passing
• The bug case / new feature is covered by tests
• Any new features are well-documented (in docstrings or notebooks)

[qiq208]

awesome, thanks!!

[avullo]

Awesome 2. Out of curiosity, why tf debugging work?

[uri-granta]

The problem is that raise gets called when you compile the function, not when you run it. Meanwhile tf.debugging.assert is similar to tf.print in that it's intended for runtime. That said, I still haven't found a clear description of the best practice re exceptions (will investigate as part of #304).