[AIRFLOW-3556] Add cross join set downstream function

[BasPH]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-3556
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.

Description

• Here are some details about my PR, including screenshots of any UI changes:

Add function to set "cross join style" downstream dependencies between two list of tasks. For example:

cross_downstream(from_tasks=[t1, t2, t3], to_tasks=[t4, t5, t6])

Sets dependencies:
    t1 --> t4
       \ /
    t2 -X> t5
       / \
    t3 --> t6

Equivalent to:
t1.set_downstream(t4)
t1.set_downstream(t5)
t1.set_downstream(t6)
t2.set_downstream(t4)
t2.set_downstream(t5)
t2.set_downstream(t6)
t3.set_downstream(t4)
t3.set_downstream(t5)
t3.set_downstream(t6)

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

HelpersTest.test_cross_downstream()

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4356 into master will increase coverage by 62%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4356      +/-   ##
==========================================
+ Coverage   16.13%   78.14%     +62%     
==========================================
  Files         202      202              
  Lines       16483    16486       +3     
==========================================
+ Hits         2660    12883   +10223     
+ Misses      13823     3603   -10220

Impacted Files	Coverage Δ
airflow/utils/helpers.py	82.6% <100%> (+55.94%)	⬆️
airflow/plugins_manager.py	93.1% <0%> (+1.14%)	⬆️
airflow/executors/dask_executor.py	2% <0%> (+2%)	⬆️
airflow/exceptions.py	100% <0%> (+2.85%)	⬆️
airflow/utils/operator_resources.py	86.95% <0%> (+4.34%)	⬆️
airflow/executors/__init__.py	63.46% <0%> (+5.76%)	⬆️
airflow/utils/module_loading.py	100% <0%> (+10.52%)	⬆️
airflow/utils/decorators.py	91.66% <0%> (+14.58%)	⬆️
airflow/settings.py	80.41% <0%> (+14.68%)	⬆️
airflow/hooks/oracle_hook.py	15.47% <0%> (+15.47%)	⬆️
... and 162 more

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[Fokko]

Thanks @BasPH