Support quantizing softmax op, test=develop (#3051)

lite/core/mir/fusion/quant_dequant_fuse_pass.cc

@@ -45,7 +45,7 @@ void QuantDequantFusePass::Apply(const std::unique_ptr<SSAGraph>& graph) {
   }
 
   // delete quant_dequant_node
-  for (auto op_type : {"pool2d", "elementwise_add"}) {
+  for (auto op_type : {"pool2d", "softmax", "elementwise_add"}) {
     fusion::DeleteQuantDequantOpFuser fuser(op_type);
     fuser(graph.get());
   }

lite/core/mir/fusion/quant_dequant_op_fuser.cc

@@ -297,7 +297,7 @@ cpp::OpDesc ChannelWiseDequantOpFuser::GenOpDesc(const key2nodes_t& matched) {
 void DeleteQuantDequantOpFuser::BuildPattern() {
   std::string quant_dequant_op_type =
       "fake_quantize_dequantize_moving_average_abs_max";
-  if (quantized_op_type_ == "pool2d") {
+  if (quantized_op_type_ == "pool2d" || quantized_op_type_ == "softmax") {
     auto* input_scale_node =
         VarNode("input_scale_node")
             ->assert_is_op_input(quant_dequant_op_type, "InScale");
@@ -374,7 +374,7 @@ void DeleteQuantDequantOpFuser::BuildPattern() {
 
 void DeleteQuantDequantOpFuser::InsertNewNode(SSAGraph* graph,
                                               const key2nodes_t& matched) {
-  if (quantized_op_type_ == "pool2d") {
+  if (quantized_op_type_ == "pool2d" || quantized_op_type_ == "softmax") {
     auto* input_scale_node = matched.at("input_scale_node");
     auto* input_act_node = matched.at("input_act_node");
     auto* quant_dequant_node = matched.at("quant_dequant_node");