[Bug Fix] Serialization of vectorActionSpace (#5220)

* [Bug Fix] Serialization of vectorActionSpace

* adding a test

com.unity.ml-agents/Runtime/Policies/BrainParameters.cs

         /// For the discrete actions: the number of branches.
         /// </value>
         [Obsolete("VectorActionSize has been deprecated, please use ActionSpec instead.")]
+        [SerializeField]
         [FormerlySerializedAs("vectorActionSize")]
         internal int[] VectorActionSize = new[] { 1 };
 
         /// (Deprecated) Defines if the action is discrete or continuous.
         /// </summary>
         [Obsolete("VectorActionSpaceType has been deprecated, please use ActionSpec instead.")]
+        [SerializeField]
         [FormerlySerializedAs("vectorActionSpaceType")]
         internal SpaceType VectorActionSpaceType = SpaceType.Discrete;
 
 #pragma warning disable CS0618
             if (!hasUpgradedBrainParametersWithActionSpec
                 && m_ActionSpec.NumContinuousActions == 0
-                && m_ActionSpec.BranchSizes == null)
+                && m_ActionSpec.NumDiscreteActions == 0)
-                    m_ActionSpec.BranchSizes = null;
-                    m_ActionSpec.NumContinuousActions = 0;
                     m_ActionSpec.BranchSizes = (int[])VectorActionSize.Clone();
                 }
             }

com.unity.ml-agents/Tests/Editor/Serialization.meta

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com.unity.ml-agents/Tests/Editor/TestModels/old_serialized_agent.prefab

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+  m_LocalScale: {x: 1, y: 1, z: 1}
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+  m_Name: 
+  m_EditorClassIdentifier: 
+  m_BrainParameters:
+    vectorObservationSize: 212
+    numStackedVectorObservations: 1
+    vectorActionSize: 27000000
+    vectorActionDescriptions: []
+    vectorActionSpaceType: 1
+  m_Model: {fileID: 11400000, guid: 4e86a19e012da43bfa5ab97ae8089b98, type: 3}
+  m_InferenceDevice: 0
+  m_BehaviorType: 0
+  m_BehaviorName: Walker
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+  m_Name: 
+  m_EditorClassIdentifier: 
+  agentParameters:
+    maxStep: 0
+  hasUpgradedFromAgentParameters: 1
+  MaxStep: 0

com.unity.ml-agents/Tests/Editor/TestModels/old_serialized_agent.prefab.meta

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com.unity.ml-agents/Tests/Editor/Serialization/TestLoadOldPrefab.cs

+using NUnit.Framework;
+using UnityEngine;
+using UnityEditor;
+using Unity.MLAgents.Policies;
+
+namespace Unity.MLAgents.Tests
+{
+    [TestFixture]
+    public class TestSerialization
+    {
+        const string k_oldPrefabPath = "Packages/com.unity.ml-agents/Tests/Editor/TestModels/old_serialized_agent.prefab";
+        const int k_numVecObs = 212;
+        const int k_numContinuousActions = 39;
+
+        [Test]
+        public void TestDeserialization()
+        {
+            var prefab = AssetDatabase.LoadAssetAtPath<GameObject>(k_oldPrefabPath);
+            var agent = GameObject.Instantiate(prefab);
+            var bp = agent.GetComponent<BehaviorParameters>();
+            Assert.AreEqual(bp.BrainParameters.ActionSpec.NumContinuousActions, k_numContinuousActions);
+            Assert.AreEqual(bp.BrainParameters.VectorObservationSize, k_numVecObs);
+        }
+
+    }
+}

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