Fix bug of nce_op

* also div num_samples when return cost of nce_op

paddle/operators/nce_op.h

@@ -197,7 +197,8 @@ class NCEGradKernel : public framework::OpKernel<T> {
     // get d_x
     auto d_x = context.Output<Tensor>(framework::GradVarName("Input"));
     if (d_x != nullptr) {
-      d_x->mutable_data<T>(context.GetPlace());
+      auto* d_x_data = d_x->mutable_data<T>(context.GetPlace());
+      std::fill(d_x_data, d_x_data + d_x->numel(), 0.0);
       auto d_x_matrix = EigenMatrix<T>::From(*d_x);
       auto w_matrix = EigenMatrix<T>::From(*(context.Input<Tensor>("Weight")));
       for (int64_t i = 0; i < sample_labels->numel(); ++i) {

python/paddle/v2/fluid/layers/nn.py

@@ -2001,9 +2001,15 @@ def nce(input,
     sample_logits = helper.create_tmp_variable(dtype=input.dtype)
     sample_labels = helper.create_tmp_variable(dtype=label.dtype)
 
-    attrs = {'num_total_classes': int(num_total_classes)}
-    if num_neg_samples is not None:
-        attrs['num_neg_samples'] = int(num_neg_samples)
+    if num_neg_samples is None:
+        num_neg_samples = 10
+    else:
+        num_neg_samples = int(num_neg_samples)
+
+    attrs = {
+        'num_total_classes': int(num_total_classes),
+        'num_neg_samples': num_neg_samples
+    }
 
     helper.append_op(
         type='nce',
@@ -2020,4 +2026,4 @@ def nce(input,
             'SampleLabels': sample_labels
         },
         attrs=attrs)
-    return cost
+    return cost / (num_neg_samples + 1)