Add Apache 2.0 license header to GradientTests

Author: nfeybesse

tensorflow-core/tensorflow-core-api/src/test/java/org/tensorflow/CustomGradientsTest.java

@@ -1,3 +1,19 @@
+/*
+ Copyright 2026 The TensorFlow Authors. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+==============================================================================
+*/
 package org.tensorflow;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;