CodeExecutionTask doesn't handle inputs correctly when in a Workflow

[shhlife]

• [ x] I have read and agree to the contributing guidelines.

Is your feature request related to a problem? Please describe.
When using a pipeline, the CodeExecutionTask can access the parent task's output like this:

def my_fn(task: CodeExecutionTask) -> BaseArtifact:
  print(task.input)
  return TextArtifact(task.input)

However, Workflows don't work this way because there are potentially multiple parent tasks. You end up needing to do this instead:
task.parents[0].output.value.

Describe the solution you'd like
It would be better if we could use task.inputs[0] instead, that way it would be more consistent with pipelines.

[collindutter]

@shhlife does this only happen with CodeExecutionTask?

[shhlife]

I'm not sure.. for regular pipeline tasks we reference the task input value as parent_output, and for workflows it's parent_outputs.

But those haven't been defined as functions the way the CodeExecutionTask needs to be