add some comments

paddle/fluid/framework/details/build_strategy.cc

@@ -27,7 +27,7 @@ class ParallelExecutorPassBuilder : public ir::PassBuilder {
  public:
   explicit ParallelExecutorPassBuilder(const BuildStrategy &strategy)
       : ir::PassBuilder(), strategy_(strategy) {
-    // Apply a graph viz pass to record a graph.
+    // Add a graph viz pass to record a graph.
     if (!strategy_.debug_graphviz_path_.empty()) {
       auto viz_pass = AppendPass("graph_viz_pass");
       const std::string graph_path = string::Sprintf(
@@ -35,10 +35,10 @@ class ParallelExecutorPassBuilder : public ir::PassBuilder {
       viz_pass->Set<std::string>("graph_viz_path", new std::string(graph_path));
     }
 
-    // Apply op fusion.
+    // Add op fusion.
     if (strategy.fuse_elewise_add_act_ops_) {
       auto fuse_elewise_add_act_pass = AppendPass("fuse_elewise_add_act_pass");
-      // Apply a graph viz pass to record a graph.
+      // Add a graph viz pass to record a graph.
       if (!strategy.debug_graphviz_path_.empty()) {
         auto viz_pass = AppendPass("graph_viz_pass");
         const std::string graph_path = string::Sprintf(
@@ -53,7 +53,7 @@ class ParallelExecutorPassBuilder : public ir::PassBuilder {
     multi_devices_pass->SetNotOwned<const BuildStrategy>("strategy",
                                                          &strategy_);
 
-    // Apply a graph print pass to record a graph with device info.
+    // Add a graph print pass to record a graph with device info.
     if (!strategy_.debug_graphviz_path_.empty()) {
       auto multi_devices_print_pass = AppendPass("multi_devices_print_pass");
       multi_devices_print_pass->SetNotOwned<const std::string>(
@@ -86,7 +86,7 @@ std::unique_ptr<ir::Graph> BuildStrategy::Apply(
 #else
     const bool use_cuda) const {
 #endif
-  // Create a default one if not intialized by user.
+  // Create a default one if not initialized by user.
   if (!pass_builder_) {
     CreatePassesFromStrategy();
   }

paddle/fluid/framework/details/build_strategy.h

@@ -69,8 +69,16 @@ struct BuildStrategy {
 
   bool enable_data_balance_{false};
 
+  // The PassBuilder assembles passes based on the configs defined above.
+  // For example, if fuse_elewise_add_act_ops_ is true, the corresponding
+  // fuse pass will be added.
+  // The PassBuilder allows for more customized insert, remove of passes
+  // from python.
+  // A new PassBuilder is created and passes are owned by the PassBuilder.
   std::shared_ptr<ir::PassBuilder> CreatePassesFromStrategy() const;
 
+  // Apply the passes built by the pass_builder_. The passes will be
+  // applied to the Program and output an ir::Graph.
   std::unique_ptr<ir::Graph> Apply(
       const ProgramDesc &main_program,
       const std::vector<platform::Place> &places,

paddle/fluid/framework/ir/pass_builder.h

@@ -28,12 +28,16 @@ class PassBuilder {
 
   virtual ~PassBuilder() {}
 
+  // Append a new pass to the end.
   std::shared_ptr<Pass> AppendPass(const std::string& pass_type);
 
+  // Insert a new pass after `idx`.
   std::shared_ptr<Pass> InsertPass(size_t idx, const std::string& pass_type);
 
+  // Remove a new pass at `idx`.
   void RemovePass(size_t idx);
 
+  // Returns a list of all passes.
   std::vector<std::shared_ptr<Pass>> AllPasses() const { return passes_; }
 
  protected: