fix wrong namespace runtime class

com.unity.render-pipelines.high-definition/HDRP/RenderPipeline/Texture2DAtlas.cs

 using System;
 using System.Collections.Generic;
 using UnityEngine.Rendering;
-using UnityEngine.Experimental.Rendering.HDPipeline;
 
 
 #if UNITY_EDITOR
-namespace UnityEngine.Experimental.Rendering
+namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
     public class AtlasAllocator
     {