Create executor.md

doc/design/executor.md

+# Executor Desgin Doc
+
+## Overview
+
+`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);
+  auto& device = device_contexts_[0];
+
+  // Instantiate all the vars in the global scope
+  for (auto& var : block.vars()) {
+    scope->NewVar(var.name());
+  }
+
+  // Decide which operator should be run
+  std::vector<bool> should_run = Preprocess(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 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);
+    }
+  }
+}
+```
+
+## Tasks
+
+As shown above, it is not hard to simply evaluate the graph. The real problem 
+is how do we actually construct the `ProgramDesc`. There are several different 
+situations that we need to consider.
+
+### 1. Init @tony @qijun
+
+##### Problem:
+
+Not sure which block to put init ops. Same concerns applys to `Load Model`.
+
+##### Solution: In seperate Blocks
+
+All `initop` and `parameter` goes to `block[0]`. Actual run starts from `block[1]`.
+
+When user writes `a = Parameter(Variable, init)`, a init op is inserted into 
+`block[0]`, and a `NOP` is inserted into  `block[1]` to substitute init op.
+
+- Pro:
+  - Init Op can be run multiple times.
+  - Compatiable with current `Executor::Preprocessing`
+  - Still only one `ProgramDesc`
+- Con:
+  - Let others know!
+
+### 2. IO
+
+#### 2.1 FeedOp and FetchOp
+
+Design Doc: https://github.com/PaddlePaddle/Paddle/pull/4599
+
+FeedOp and FetchOp in distributed environment: 
+https://github.com/PaddlePaddle/Paddle/issues/4613
+
+#### 2.2 ReaderOp and WriterOp
+
+### 3. Backward @jiayi
+
+Executor test case is a good place to test `backward` module, even though executor 
+is not necessarily depends on `backward`. Currently exposed issue:
+
+- Fill One: https://github.com/PaddlePaddle/Paddle/issues/4627
+- Attribute map: https://github.com/PaddlePaddle/Paddle/issues/4642
+
+### 4. Optimizer @longfei
+
+Executor test case is a good place to test `optimizer `module, even though executor 
+is not necessarily depends on `optimizer `.
+
+### 5. RNN @chunwei
+
+To be discussed.
+
+- How to deal with multiple blocks
+- How to deal with LoDTensor
+