Allow implicit n-arry outputs of chain composite and pipelines.

sdks/python/apache_beam/yaml/yaml_transform.py

@@ -518,10 +518,13 @@ class CompositePTransform(beam.PTransform):
     @staticmethod
     def expand(inputs):
       inner_scope.compute_all()
-      return {
-          key: inner_scope.get_pcollection(value)
-          for (key, value) in spec['output'].items()
-      }
+      if '__implicit_outputs__' in spec['output']:
+        return inner_scope.get_outputs(spec['output']['__implicit_outputs__'])
+      else:
+        return {
+            key: inner_scope.get_pcollection(value)
+            for (key, value) in spec['output'].items()
+        }
 
   if 'name' not in spec:
     spec['name'] = 'Composite'
@@ -596,7 +599,7 @@ def is_not_output_of_last_transform(new_transforms, value):
         for (key, value) in composite_spec['output'].items()
     }
   else:
-    composite_spec['output'] = last_transform
+    composite_spec['output'] = {'__implicit_outputs__': last_transform}
   if 'name' not in composite_spec:
     composite_spec['name'] = 'Chain'
   composite_spec['type'] = 'composite'