Issue 4120 (#4297)

* fix issue 4120, add reasonable exception when user try to use OnnxSequenceType attribute without specify sequence type

* rename function for better readibility

* take comments and modify accordingly

* take eric's comments, comment out default constructor instead of throw exception

* refine comments

* remove unnecessary code change

* Make default constructor of OnnxSequenceType attribute obsolete

* hide default constructor of OnnxSequenceType attribute from intellisense and make obsolete not cause compiler error just to be safe

src/Microsoft.ML.OnnxTransformer/OnnxSequenceType.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using Microsoft.ML.Data;
 
 namespace Microsoft.ML.Transforms.Onnx
@@ -58,8 +59,9 @@ public sealed class OnnxSequenceTypeAttribute : DataViewTypeAttribute
 
         // Make default constructor obsolete.
         // Use default constructor will left the _elemType field empty and cause exception in methods using _elemType.
-        // User will receive compile error when try to use [OnnxSequenceType] attribute directly without specify sequence type
-        [Obsolete("Please specify sequence type when use OnnxSequenceType Attribute", true)]
+        // User will receive compile warning when try to use [OnnxSequenceType] attribute directly without specify sequence type
+        [Obsolete("Please specify sequence type when use OnnxSequenceType Attribute", false)]
+        [EditorBrowsable(EditorBrowsableState.Never)]
         public OnnxSequenceTypeAttribute()
         {
         }