Fix fail when a variable receives zero gradient #482 (#483)

Author: ramon-garcia

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

@@ -147,6 +147,15 @@ Pointer getUnsafeNativeHandle() {
     return operation.getUnsafeNativeHandle(index);
   }
 
+  /**
+   * Returns whether the underlying operation has no valid handle. Makes the opposite check as
+   * GraphOperation.requireHandle *
+   */
+  public boolean isClosed() {
+    Pointer handle = operation.getUnsafeNativeHandle(index);
+    return handle == null || handle.isNull();
+  }
+
   private final AbstractOperation operation;
   private final int index;
 }

tensorflow-framework/src/main/java/org/tensorflow/framework/optimizers/Optimizer.java

@@ -174,7 +174,9 @@ public Op applyGradients(List<GradAndVar<? extends TType>> gradsAndVars, String
     List<Op> updateOps = new ArrayList<>();
     prepOp.ifPresent(updateOps::add);
     for (GradAndVar<? extends TType> pair : gradsAndVars) {
-      updateOps.add(applyDense(pair));
+      if (!pair.gradient.isClosed()) {
+        updateOps.add(applyDense(pair));
+      }
     }
 
     return finish(updateOps, name);