converted AssetSourceBase to an internal interface

com.unity.perception/Editor/Randomization/Utilities/StaticData.cs

             randomizerTypes = GetConstructableDerivedTypes<Randomizer>();
             samplerTypes = GetConstructableDerivedTypes<ISampler>();
             assetSourceLocationTypes = GetConstructableDerivedTypes<AssetSourceLocation>();
-            assetRoleTypes = GetConstructableDerivedTypes<AssetRoleBase>();
+            assetRoleTypes = GetConstructableDerivedTypes<IAssetRoleBase>();
         }
 
         static Type[] GetConstructableDerivedTypes<T>()

com.unity.perception/Editor/Randomization/VisualElements/AssetSource/AssetSourceElement.cs

         Type m_AssetType;
         Dictionary<string, Type> m_AssetRoleLabelsToTypes = new Dictionary<string, Type>();
 
-        AssetRoleBase assetRole =>
-            (AssetRoleBase)StaticData.GetManagedReferenceValue(m_AssetRoleProperty);
+        IAssetRoleBase assetRole =>
+            (IAssetRoleBase)StaticData.GetManagedReferenceValue(m_AssetRoleProperty);
         AssetSourceLocation assetSourceLocation =>
             (AssetSourceLocation)StaticData.GetManagedReferenceValue(m_LocationProperty);
 
             else
             {
                 m_AssetRoleToolbarMenu.text = GetAssetRoleDisplayName(type);
-                var newAssetRole = (AssetRoleBase)Activator.CreateInstance(type);
+                var newAssetRole = (IAssetRoleBase)Activator.CreateInstance(type);
                 m_AssetRoleProperty.managedReferenceValue = newAssetRole;
             }
             m_AssetRoleProperty.serializedObject.ApplyModifiedProperties();
 
         static string GetAssetRoleDisplayName(Type type)
         {
-            return $"{((AssetRoleBase)Activator.CreateInstance(type)).label} ({type.Name})";
+            return $"{((IAssetRoleBase)Activator.CreateInstance(type)).label} ({type.Name})";
         }
     }
 }

com.unity.perception/Runtime/Randomization/Randomizers/AssetSources/AssetRole.cs

     /// Typed asset roles are used to apply preprocessing steps to assets loaded from an <see cref="AssetSource{T}"/>.
     /// </summary>
     /// <typeparam name="T">The type of asset to preprocess</typeparam>
-    public abstract class AssetRole<T> : AssetRoleBase where T : Object
+    public abstract class AssetRole<T> : IAssetRoleBase where T : Object
+        /// <inheritdoc/>
+        public abstract string label { get; }
+
+        /// <inheritdoc/>
+        public abstract string description { get; }
+
         /// <summary>
         /// Perform preprocessing operations on an asset loaded from an <see cref="AssetSource{T}"/>.
         /// </summary>

com.unity.perception/Runtime/Randomization/Randomizers/AssetSources/AssetSource.cs

     [Serializable]
     public sealed class AssetSource<T> where T : Object
     {
-        [SerializeReference] AssetRoleBase m_AssetRoleBase;
+        [SerializeReference] IAssetRoleBase m_AssetRoleBase;
 
         /// <summary>
         /// The location to load assets from

com.unity.perception/Runtime/Randomization/Randomizers/AssetSources/IAssetRoleBase.cs.meta

 fileFormatVersion: 2
-guid: 1af66eeef6ffa514a8307a228b8e85f9
+guid: 7c335d3150918dc49be9cf2eddb144d2
 MonoImporter:
   externalObjects: {}
   serializedVersion: 2

com.unity.perception/Runtime/Randomization/Randomizers/AssetSources/IAssetRoleBase.cs

     /// <summary>
     /// The base asset role class. Derive from <see cref="AssetRole{T}"/> instead to create a new asset role.
     /// </summary>
-    public abstract class AssetRoleBase
+    interface IAssetRoleBase
     {
         /// <summary>
         /// The string label uniquely associated with this asset role