Retry for TaskGroup

### Discussed in https://github.com/apache/airflow/discussions/21333 <div type='discussions-op-text'> ^{Originally posted by **sartyukhov** February 4, 2022} ### Description Hello! Previously, a SubDag was used to organize tasks into groups. Now you've introduced a TaskGroups to the world . It's nice and very clever. But it has a one big disadvantage over the SubDag - it cant be repeated. ### Use case/motivation For example: In a project I have two task (A >> B): A - collect data (PythonOperator) B - update material view in postgres (PostgresOperator) 'A' could collect only part of data and mark itself as failed (there is no "half-failed" status as I know). But task 'B' should run regardless of A`s result (trigger_rule="all_done" for example) to update matview with part of data. In an ~ hour I would like to repeat that process (A >> B). With SubDag I could do that: - initiate SubDag with parameter _retries=10_ - add DummyTask 'C' with trigger_rule="all_success" - change flow to A >> B >> C and A >> C and that's it, C marks dag as failed and trigger it to retry. But TaskGroup does not have retry parameter. I also can't retry whole DAG, because it's big. I also don't want to update material view inside task 'A' because in that way I can't do [A0, A1..An] >> B (update material view just once for several collects). I hope it's possible. Or maybe it could be done some other way. Thanks in advance. # Additional explanation on the use case (from #21333) I have a specific use case where this feature would be useful. It is like: There is a task to do one thing There a second task (which depends on the first one) that does another thing, if this one fails I'll need to re-run the entire dag. I can't do both processes in the same task due to some limitations (I work with different java drivers on each one) and retrying the same task doesn't solve the problem because the result of this task will imply whether or not the first dag would need a re-execution. Clear the pr…