larger network/6 stacked obs

Project/Assets/ML-Agents/Examples/Tennis/Prefabs/TennisArea.prefab

   m_EditorClassIdentifier: 
   m_BrainParameters:
     vectorObservationSize: 10
-    numStackedVectorObservations: 3
+    numStackedVectorObservations: 6
     vectorActionSize: 03000000
     vectorActionDescriptions: []
     vectorActionSpaceType: 1
   m_EditorClassIdentifier: 
   m_BrainParameters:
     vectorObservationSize: 10
-    numStackedVectorObservations: 3
+    numStackedVectorObservations: 6
     vectorActionSize: 03000000
     vectorActionDescriptions: []
     vectorActionSpaceType: 1

config/trainer_config.yaml

     learning_rate_schedule: constant
     batch_size: 2048
     buffer_size: 20480
-    hidden_units: 256
+    hidden_units: 512
     time_horizon: 1000
     beta: 1.0e-2
     self_play: