Update executor.md

doc/design/executor.md

@@ -5,8 +5,8 @@
 `Executor` evaluates a `ProgramDesc`. Essentially, it instantializes Variables and Operators, then run all the operators
 
 ```c++
-void Executor::Run(const ProgramDesc& pdesc, Scope* scope) {
-  auto& block = pdesc.blocks(0);
+void Executor::Run(const ProgramDesc& pdesc, Scope* scope, int block_id) {
+  auto& block = pdesc.blocks(block_id);
   auto& device = device_contexts_[0];
 
   // Instantiate all the vars in the global scope
@@ -14,55 +14,19 @@ void Executor::Run(const ProgramDesc& pdesc, Scope* scope) {
     scope->NewVar(var.name());
   }
 
-  // Decide which operator should be run
-  std::vector<bool> should_run = Prune(pdesc);
-
   // Run the block
   Scope& local_scope = scope->NewScope();
   for (size_t i = 0; i < should_run.size(); ++i) {
-    if (should_run[i]) {
-      for (auto var : block.ops(i).outputs()) {
-        for (auto argu : var.arguments()) {
-          // Create temp variable in the local_scope
-          if (local_scope.FindVar(argu) == nullptr) {
-            local_scope.NewVar(argu);
-          }
+    for (auto var : block.ops(i).outputs()) {
+      for (auto argu : var.arguments()) {
+        // Create temp variable in the local_scope
+        if (local_scope.FindVar(argu) == nullptr) {
+          local_scope.NewVar(argu);
         }
       }
-      auto op = paddle::framework::OpRegistry::CreateOp(block.ops(i));
-      op->Run(local_scope, *device);
     }
+    auto op = paddle::framework::OpRegistry::CreateOp(block.ops(i));
+    op->Run(local_scope, *device);
   }
 }
 ```
-
-## Challenge
-
-It is not hard to simply evaluate a graph. However, it is hard to determine which op should be run. Consider the following different situations.
-
-```python
-# Case 1: run foward pass.
-cost_np = executor.run(target=cost)
-# Case 2: run backward passing.
-opts_np, _ = executor.run(target=[cost, opt])
-# Case 3: run checkpointing
-_ = executor.run(target=checkpoint)
-```
-
-We want to support the evaluation of both variables and operators.
-
-## Solution
-
-To support evaluation of operators, we add `is_target` field in the `OpDesc`.
-
-```c++
-message OpDesc {
-  required string type = 3;
-  repeated Var inputs = 1;
-  repeated Var outputs = 2;
-  repeated Attr attrs = 4;
-  required bool is_target = 5 [ default = false ]; // true if the op is target
-};
-```
-
-To support evaluation of variables, we add [fetch_op](https://github.com/PaddlePaddle/Paddle/pull/4599). For each variable in the `target`, we insert a `fetch_op` into the `ProgramDesc`. (Also, a user may want to overwrite a variable, so we also added [feed_op](https://github.com/PaddlePaddle/Paddle/pull/4599). )