Replace std::expf with expf since expf is not a std function for ubuntu os

test/operators/test_sequence_softmax_op.cpp

@@ -34,7 +34,7 @@ void SequenceSoftmax(const framework::LoDTensor *X, framework::LoDTensor *Y) {
     }
     float sum = 0.f;
     for (int j = 0; j < num_classes; ++j) {
-      float tmp = std::expf(input[j] - max);
+      float tmp = expf(input[j] - max);
       sum += tmp;
       output[j] = tmp;
     }

test/operators/test_softmax_op.cpp

@@ -38,7 +38,7 @@ void Softmax(const framework::Tensor *X, framework::Tensor *Y) {
       }
       float sum = 0.f;
       for (int j = 0; j < num_classes; ++j) {
-        float tmp = std::expf(input[j] - max);
+        float tmp = expf(input[j] - max);
         sum += tmp;
         output[j] = tmp;
       }