fix(sdk): Prevents dsl.ParallelFor over single parameter from compiling.

[KevinGrantLee]

Description of your changes:

Prevents the following pipeline from compiling

@dsl.component
def str_identity(s: str) -> str:
    return s


@dsl.pipeline
def my_pipeline():
  single_param = str_identity(s='string')
  with dsl.ParallelFor(items=single_param.output) as item:
      str_identity(s=item)

The component str_identity outputs a str so routing this to dsl.ParallelFor does not make sense.

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[connor-mccarthy]

Thank you, @KevinGrantLee!

[connor-mccarthy]

Thanks, Kevin!

[connor-mccarthy]

/lgtm
/approve

Thank you, @KevinGrantLee!

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: connor-mccarthy

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[KevinGrantLee]

/retest

[KevinGrantLee]

/retest-required

[KevinGrantLee]

/retest

[google-oss-prow]

@KevinGrantLee: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kfp-kubernetes-execution-tests	d50d1e7	link	false	/test kfp-kubernetes-execution-tests

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