Flattening multiple outputs of a ParDoN fails

If a user does a beam.ParDoN for pardo > 2  and then passes one or more of the outputs to a flatten, then if the flatten occurs SDK side, it currently creates multiple flatten nodes, which then triggers the downstream pardo (the DoFn that consumes the Flatten's output) to be initialized multiple times for a single bundle.

The fix is to pre-emptively populate the input links with the first created flatten, so subsequent tracings of the plan use the same flatten node the same way the Go direct runner does[1]. That would happen in the exec translate code.

[1] https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/runners/direct/direct.go#L299

[2] https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/exec/translate.go#L493