s_m_l tasks should not require you to comment out parts of a task in order to retry it.

So for example:

1) If you run EmptyShardAddToProdAndVerifyTask a second time, it might fail at the "atest server add" step because the server is already in prod. It should not do that.

2) Or it might fail at the atest shard add_boards step because the dummy board label you added to that shard was already added to it. It should not do that.