input_vars of flatbuffers impl, test=develop (#4147)

* input_vars of flatbuffers impl, test=develop

* add comments in build.sh, test=develop

lite/model_parser/flatbuffers/op_desc.h

@@ -171,8 +171,15 @@ class OpDescView : public OpDescAPI {
   }
 
   std::vector<std::string> input_vars() const {
-    NotImplemented();
-    return std::vector<std::string>();
+    VLOG(5) << "This function call is expensive.";
+    std::vector<std::string> res;
+    for (const auto& var : *(desc_->inputs())) {
+      if (var && var->arguments()) {
+        res.emplace_back(var->arguments()->begin()->c_str(),
+                         var->arguments()->end()->c_str());
+      }
+    }
+    return res;
   }
 
   std::vector<std::string> output_vars() const {

lite/tools/build.sh

@@ -436,6 +436,7 @@ function print_usage {
     echo -e "--build_python: (OFF|ON); controls whether to publish python api lib (ANDROID and IOS is not supported)"
     echo -e "--build_java: (OFF|ON); controls whether to publish java api lib (Only ANDROID is supported)"
     echo -e "--build_dir: directory for building"
+    echo -e "--enable_flatbuffers_view: (OFF|ON); Use the flatbuffers read-only view to load the model. If ON, the naive buffer will no longer be supported."
     echo
     echo -e "argument choices:"
     echo -e "--arm_os:\t android|ios|ios64"