Add documentation about dangerousGradientBuilder

Signed-off-by: Ryan Nett <JNett96@gmail.com>

Author: rnett

tensorflow-core/tensorflow-core-api/src/main/java/org/tensorflow/BaseGradientAdapter.java

@@ -78,6 +78,12 @@ protected static GraphOperation getGraphOp(Graph g, Node node) {
     }
   }
 
+  /**
+   * Use builders without locking.  This should only be used during custom gradient building.
+   *
+   * The graph locks are not re-entrant, so attempting to add an op to a graph that has
+   * been locked by the gradient builder will fail without this.
+   */
   protected static void useDangerousLockedBuilders(Graph g, boolean dangerous) {
     g.setDangerousGradientBuilder(dangerous);
   }

tensorflow-core/tensorflow-core-api/src/main/java/org/tensorflow/Graph.java

@@ -904,6 +904,12 @@ synchronized SaverDef saverDef() {
   private final Set<Operation> initializers = Collections.synchronizedSet(new LinkedHashSet<>());
   private boolean newInitializers = false;
 
+  /**
+   * Use builders without locking.  This should only be used during custom gradient building.
+   *
+   * The graph locks are not re-entrant, so attempting to add an op to a graph that has
+   * been locked by the gradient builder will fail without this.
+   */
   synchronized void setDangerousGradientBuilder(boolean dangerous) {
     dangerousGradientBuilder = dangerous;
   }

tensorflow-core/tensorflow-core-api/src/main/java/org/tensorflow/GraphOperationBuilder.java

@@ -400,6 +400,13 @@ public OperationBuilder setAttr(String name, AttrValue value) {
   private TF_OperationDescription unsafeNativeHandle;
   private final Graph graph;
   private final Scope scope;
+
+  /**
+   * Use builders without locking.  This should only be used during custom gradient building.
+   *
+   * The graph locks are not re-entrant, so attempting to add an op to a graph that has
+   * been locked by the gradient builder will fail without this.
+   */
   private final boolean dangerousGradientBuilder;
 
   private static void requireHandle(Pointer handle) {