Assign new BrainParametersProto fields based on capabilities (#4581)

com.unity.ml-agents/Runtime/Actuators/ActionSpec.cs

         {
             if (NumContinuousActions > 0 && NumDiscreteActions > 0)
             {
-                throw new UnityAgentsException("ActionSpecs must be all continuous or all discrete.");
+                throw new UnityAgentsException("Hybrid action spaces not supported by the trainer. " +
+                    "ActionSpecs must be all continuous or all discrete.");
             }
         }
     }

com.unity.ml-agents/Runtime/Communicator/GrpcExtensions.cs

         /// <param name="isTraining">Whether or not the Brain is training.</param>
         public static BrainParametersProto ToBrainParametersProto(this ActionSpec actionSpec, string name, bool isTraining)
         {
-            actionSpec.CheckNotHybrid();
-
-            if (actionSpec.NumContinuousActions > 0)
+            var actionSpecProto = new ActionSpecProto
-                brainParametersProto.VectorActionSizeDeprecated.Add(actionSpec.NumContinuousActions);
-                brainParametersProto.VectorActionSpaceTypeDeprecated = SpaceTypeProto.Continuous;
-            }
-            else if (actionSpec.NumDiscreteActions > 0)
+                NumContinuousActions = actionSpec.NumContinuousActions,
+                NumDiscreteActions = actionSpec.NumDiscreteActions,
+                DiscreteBranchSizes = { actionSpec.BranchSizes },
+            };
+            brainParametersProto.ActionSpec = actionSpecProto;
+
+            var supportHybrid = Academy.Instance.TrainerCapabilities == null || Academy.Instance.TrainerCapabilities.HybridActions;
+            if (!supportHybrid)
-                brainParametersProto.VectorActionSizeDeprecated.AddRange(actionSpec.BranchSizes);
-                brainParametersProto.VectorActionSpaceTypeDeprecated = SpaceTypeProto.Discrete;
+                actionSpec.CheckNotHybrid();
+                if (actionSpec.NumContinuousActions > 0)
+                {
+                    brainParametersProto.VectorActionSizeDeprecated.Add(actionSpec.NumContinuousActions);
+                    brainParametersProto.VectorActionSpaceTypeDeprecated = SpaceTypeProto.Continuous;
+                }
+                else if (actionSpec.NumDiscreteActions > 0)
+                {
+                    brainParametersProto.VectorActionSizeDeprecated.AddRange(actionSpec.BranchSizes);
+                    brainParametersProto.VectorActionSpaceTypeDeprecated = SpaceTypeProto.Discrete;
+                }
             }
 
             // TODO handle ActionDescriptions?