Fixes #4571. About memory leak when using FeaturizeText.

src/Microsoft.ML.Core/Utilities/NormStr.cs

@@ -29,7 +29,7 @@ internal sealed class NormStr
         private NormStr(ReadOnlyMemory<char> str, int id, uint hash)
         {
             Contracts.Assert(id >= 0 || id == -1 && str.IsEmpty);
-            Value = str;
+            Value = new ReadOnlyMemory<char>(str.ToString().ToCharArray());
             Id = id;
             _hash = hash;
         }

src/Microsoft.ML.Transforms/Text/TextNormalizing.cs

@@ -398,7 +398,8 @@ private void NormalizeSrc(in ReadOnlyMemory<char> src, ref ReadOnlyMemory<char>
                 if (min == 0)
                 {
                     Host.Assert(src.Length == len);
-                    dst = src;
+                    buffer.AppendSpan(span);
+                    dst = buffer.ToString().AsMemory();
                 }
                 else
                 {