Add sample test for multiple output

[gaoning777]

This change is 

[Ark-kun]

Thank you. Sorry for not doing this sooner. I was busy revamping other samples and the sample test system.

[Ark-kun]

I see the following error in the tests:
Is there any particular reason to require the GCS path to end with a letter or number?

run-sample-tests-loop(19:multiple_outputs):	     36     # The first and las characters must be alphanumeric.
run-sample-tests-loop(19:multiple_outputs):	     37     if not all([name[0].isalnum(), name[-1].isalnum()]):
run-sample-tests-loop(19:multiple_outputs):	---> 38         raise ValueError("Bucket names must start and end with a number or letter.")
run-sample-tests-loop(19:multiple_outputs):	     39     return name
run-sample-tests-loop(19:multiple_outputs):	     40 
run-sample-tests-loop(19:multiple_outputs):	ValueError: Bucket names must start and end with a number or letter.

[gaoning777]

Please review @Ark-kun

[gaoning777]

fixing #1813

[Ark-kun]

/lgtm
P.S. The fix for multiline decorators is here: #2345

[Ark-kun]

/lgtm

[gaoning777]

/approve

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[monicatao]

Hi How can I create multiple outputs with dsl.ContainerOp function without saving the results in file_outputs?