Issue 4120, add reasonable exception when user try to use OnnxSequenceType attribute without specify sequence type (dotnet#4272)

* 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

Author: frank-dong-ms-zz

src/Microsoft.ML.OnnxTransformer/OnnxSequenceType.cs

@@ -56,9 +56,10 @@ public sealed class OnnxSequenceTypeAttribute : DataViewTypeAttribute
     {
         private Type _elemType;
 
-        /// <summary>
-        /// Create a sequence type.
-        /// </summary>
+        // 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)]
         public OnnxSequenceTypeAttribute()
         {
         }

test/Microsoft.ML.Tests/OnnxSequenceTypeWithAttributesTest.cs

@@ -65,4 +65,4 @@ public void OnnxSequenceTypeWithColumnNameAttributeTest()
 
         }
     }
-}
+}