Fix gradients with ignore_idx in softmax_with_cross_entropy on cpu.

Remove softmax_with_cross_entropy from op_threshold_white_list.
test=develop

Author: guoshengCS

paddle/fluid/operators/softmax_with_cross_entropy_op.h

@@ -116,12 +116,13 @@ class SoftmaxWithCrossEntropyGradKernel : public framework::OpKernel<T> {
       for (int i = 0; i < n; ++i) {
         for (int j = 0; j < remain; j++) {
           int idx = i * remain + j;
-          logit_grad_data[i * d + label_data[idx] * remain + j] -=
-              out_grad_data[idx];
           if (label_data[idx] == ignore_index) {
             for (int k = 0; k < axis_dim; ++k) {
               logit_grad_data[i * d + k * remain + j] = 0;
             }
+          } else {
+            logit_grad_data[i * d + label_data[idx] * remain + j] -=
+                out_grad_data[idx];
           }
         }
       }

python/paddle/fluid/tests/unittests/test_softmax_with_cross_entropy_op.py

@@ -83,17 +83,17 @@ def setUp(self):
         self.attrs = {
             "numeric_stable_mode": self.numeric_stable_mode,
             "soft_label": self.soft_label,
+            "ignore_index": self.ignore_index,
         }
-        if self.ignore_index >= 0:
-            self.attrs['ignore_index'] = self.ignore_index
+
         if self.axis != -1:
             self.attrs['axis'] = self.axis
 
     def test_check_output(self):
         self.check_output()
 
     def test_check_grad(self):
-        self.check_grad(["Logits"], "Loss", max_relative_error=0.05)
+        self.check_grad(["Logits"], "Loss")
 
 
 class TestSoftmaxWithCrossEntropyOpNoCudnn(TestSoftmaxWithCrossEntropyOp):

python/paddle/fluid/tests/unittests/white_list/op_threshold_white_list.py

@@ -36,7 +36,6 @@
     'selu', \
     'sigmoid_cross_entropy_with_logits', \
     'soft_relu', \
-    'softmax_with_cross_entropy', \
     'spp', \
     'teacher_student_sigmoid_loss', \
     'unpool', \