Parser.clear() does not clear until session.commit() is executed

[HiromuHota]

Description of the bug

Parser.clear() does not clear anything until session.commit() is executed elsewhere.
According to https://docs.sqlalchemy.org/en/13/orm/session_basics.html#deleting,

The Session.delete() method places an instance into the Session’s list of objects to be marked as deleted

To Reproduce

Steps to reproduce the behavior:

>>> corpus_parser.apply(doc_preprocessor)
>>> corpus_parser.clear()
>>> assert session.query(Document).count() == 0

Expected behavior

When corpus_parser.clear() above is executed, any instance of Context like Document, Sentence should be deleted.

Error Logs/Screenshots

If applicable, add error logs or screenshots to help explain your problem.

Environment (please complete the following information)

• Fonduer Version: 0.8.2

Additional context

Add any other context about the problem here.

This might be related to #213

[HiromuHota]

I'm trying to write a unit test that can reproduce this issue, but can't.
Close for now.