Fixing multi-sender stage configurations

[mdemoret-nv]

Description

This PR fixes a long overdue placeholder exception from back when we were still using streamz. If a user configured multiple upstream stages to a single downstream port, then you would get a NotImplementedError. For example, the following would cause an exception:

source1 = pipe.add_stage(InMemorySourceStage(config, [filter_probs_df]))
source2 = pipe.add_stage(InMemorySourceStage(config, [filter_probs_df]))

sink_stage = pipe.add_stage(InMemorySinkStage(config))

pipe.add_edge(source1, sink_stage)
pipe.add_edge(source2, sink_stage)

Also adds a simple tests to run the above scenario.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mdemoret-nv]

Discovered when reviewing PR #939

[dagardner-nv]

LGTM

[drobison00]

Looks good and a really useful functionality bump.

[mdemoret-nv]

/merge