Reduce risks of errors due to `CreateProcessingJob` supporting only strings

[AdrianB-sovo]

When trying to reference a pipeline parameter of type ParameterInteger when creating a pipeline job, I get the following error: botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the CreatePipeline operation: Cannot assign property reference [Parameters.split_number] to argument of type [String].

I saw that it wasn't supported and that it's not expected to change (https://github.com/aws/sagemaker-python-sdk/issues/2631). Since it could prevent people — like me — from searching what they did wrong when it's just not supported, I think it should be better documented.

I see two ways to improve the documentation:

• Change the type hints of arguments for the steps.

From:

arguments: Optional[List[Union[str, PipelineVariable]]] = None

to

arguments: Optional[List[Union[str, ParameterString]]] = None

• Add comments explaining that CreateProcessingJob API only supports strings, so any other type of PipelineVariable should be converted.