Bug fix fetch_callback of Partial Subset DAG

[syun64]

I believe that there may be a bug where if a partial_subset dag is generated on a DAG with on_success_callback or on_failure_callback functions defined on it, then it might try to look for a TI that is not actually a part of the partial subset DAG. This is because we fetch the last TI available from the DagRun within DAG.fetch_callback, and a DagRun isn't aware of the concept of a partial subset. Hence ti.task = dag.get_task(ti.task_id) results in a TaskNotFound exception.

In this PR, we are:

1. Adding DAG callback functions to the test DAG defined in tests/jobs/test_backfill_job.py::TestBackfillJob::test_sub_set_subdag to cover this test case (which fails without below patch)
2. If dag.partial, we filter out tis where ti.state == State.NONE within DAG.fetch_callback

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[syun64]

Thank you for the review @vincbeck . Would you be able to merge this in? Unfortunately I do not have write access to the repository

[vincbeck]

Done! Thanks for the PR!