Fix sparse_categorical_crossentropy.

src/TensorFlowNET.Keras/BackendImpl.cs

@@ -307,9 +307,8 @@ namespace Tensorflow.Keras
             var update_shape = target_rank > -1 && output_rank > -1 && target_rank != output_rank - 1;
             if (update_shape)
             {
-                /*var target = flatten(target);
-                output = tf.reshape(output, [-1, output_shape[-1]]);*/
-                throw new NotImplementedException("");
+                target = tf.reshape(target, -1);
+                output = tf.reshape(output, (-1, output.shape[-1]));
             }
 
             if (ignore_class.HasValue)

test/TensorFlowNET.Keras.UnitTest/MultiThreadsTest.cs

@@ -8,7 +8,7 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 
 namespace TensorFlowNET.Keras.UnitTest
 {
-    [TestClass, Ignore]
+    [TestClass]
     public class MultiThreads
     {
         [TestMethod]