Pipeline with a single dangling estimator node triggers an exception

[raghukiran1224]

Describe the bug Possibly a corner case? ray-pipeline/codeflare/pipelines/Datamodel.py in get_pre_edges(self, node) 640 """ 641 pre_edges = [] --> 642 pre_nodes = self.__pre_graph__[node] 643 # Empty pre 644 if not pre_nodes:

KeyError: <codeflare.pipelines.Datamodel.EstimatorNode object at 0x7fa2d8920f10>

To Reproduce

## initialize codeflare pipeline by first creating the nodes
pipeline = dm.Pipeline()
node_a = dm.EstimatorNode('a', MinMaxScaler())
node_b = dm.EstimatorNode('b', StandardScaler())
node_c = dm.EstimatorNode('c', MaxAbsScaler())
node_d = dm.EstimatorNode('d', RobustScaler())

node_e = dm.AndNode('e', FeatureUnion())
node_f = dm.AndNode('f', FeatureUnion())
node_g = dm.AndNode('g', FeatureUnion())

## codeflare nodes are then connected by edges
pipeline.add_edge(node_a, node_e)
pipeline.add_edge(node_b, node_e)
pipeline.add_edge(node_c, node_f)
## node_d does not have a downstream node
# pipeline.add_edge(node_d, node_f)
pipeline.add_edge(node_e, node_g)
pipeline.add_edge(node_f, node_g)

pipeline_input = dm.PipelineInput()
xy = dm.Xy(X,y)
pipeline_input.add_xy_arg(node_a, xy)
pipeline_input.add_xy_arg(node_b, xy)
pipeline_input.add_xy_arg(node_c, xy)
pipeline_input.add_xy_arg(node_d, xy)

## execute the codeflare pipeline
pipeline_output = rt.execute_pipeline(pipeline, ExecutionType.FIT, pipeline_input)

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[raghukiran1224]

closed per #31