fix RNN and IfElse syntax in Block design (#4210)

* fix block syntax

* add builder

* fix argument names

* uniform all Var to var

* fix c demo

* factor set_outputs

* update block

* delete as_ifelse_input interface from block design

* simplify set_outputs and output_num

* make the c++ codes compatible with demo

* fix compatible

* fix syntax

* update ie

* update syntax

doc/design/block.md

@@ -55,17 +55,23 @@ Let us consolidate the discussion by presenting some examples.
 The following C++ programs shows how blocks are used with the `if-else` structure:
 
 ```c++
+namespace pd = paddle;
+
 int x = 10;
-int y = 20;
-int out;
+int y = 1;
+int z = 10;
 bool cond = false;
+int o1, o2;
 if (cond) {
   int z = x + y;
-  out = softmax(z);
+  o1 = z;
+  o2 = pd::layer::softmax(z);
 } else {
-  int z = fc(x);
-  out = z;
+  int d = pd::layer::fc(z);
+  o1 = d;
+  o2 = d+1;
 }
+
 ```
 
 An equivalent PaddlePaddle program from the design doc of the [IfElseOp operator](./if_else_op.md) is as follows:
@@ -73,57 +79,55 @@ An equivalent PaddlePaddle program from the design doc of the [IfElseOp operator
 ```python
 import paddle as pd
 
-x = var(10)
-y = var(20)
-cond = var(false)
-ie = pd.create_ifelseop(inputs=[x], output_num=1)
+x = minibatch([10, 20, 30]) # shape=[None, 1]
+y = var(1) # shape=[1], value=1
+z = minibatch([10, 20, 30]) # shape=[None, 1]
+cond = larger_than(x, 15) # [false, true, true]
+
+ie = pd.ifelse()
 with ie.true_block():
-    x = ie.inputs(true, 0)
-    z = operator.add(x, y)
-    ie.set_output(true, 0, operator.softmax(z))
+    d = pd.layer.add_scalar(x, y)
+    ie.output(d, pd.layer.softmax(d))
 with ie.false_block():
-    x = ie.inputs(false, 0)
-    z = layer.fc(x)
-    ie.set_output(true, 0, operator.softmax(z))
-out = b(cond)
+    d = pd.layer.fc(z)
+    ie.output(d, d+1)
+o1, o2 = ie(cond)
 ```
 
-In both examples, the left branch computes `softmax(x+y)` and the right branch computes `fc(x)`.
+In both examples, the left branch computes `x+y` and `softmax(x+y)`, the right branch computes `x+1` and `fc(x)`.
 
 A difference is that variables in the C++ program contain scalar values, whereas those in the PaddlePaddle programs are mini-batches of instances.  The `ie.input(true, 0)` invocation returns instances in the 0-th input, `x`, that corresponds to true values in `cond` as the local variable `x`, where `ie.input(false, 0)` returns instances corresponding to false values.
 
+
 ### Blocks with `for` and `RNNOp`
 
 The following RNN model from the [RNN design doc](./rnn.md)
 
 ```python
-x = sequence([10, 20, 30])
-m = var(0)
-W = tensor()
-U = tensor()
-
-rnn = create_rnn(inputs=[input])
-with rnn.stepnet() as net:
-  x = net.set_inputs(0)
-  h = net.add_memory(init=m)
-  fc_out = pd.matmul(W, x)
-  hidden_out = pd.matmul(U, h.pre(n=1))
-  sum = pd.add_two(fc_out, hidden_out)
-  act = pd.sigmoid(sum)
-  h.update(act)                       # update memory with act
-  net.set_outputs(0, act, hidden_out) # two outputs
-
+x = sequence([10, 20, 30]) # shape=[None, 1]
+m = var(0) # shape=[1]
+W = var(0.314, param=true) # shape=[1]
+U = var(0.375, param=true) # shape=[1]
+
+rnn = pd.rnn()
+with rnn.step():
+  h = rnn.memory(init = m)
+  hh = rnn.previous_memory(h)
+  a = layer.fc(W, x)
+  b = layer.fc(U, hh)  
+  s = pd.add(a, b)
+  act = pd.sigmoid(s)
+  rnn.update_memory(h, act)
+  rnn.output(a, b)
 o1, o2 = rnn()
-print o1, o2
 ```
-
 has its equivalent C++ program as follows
 
 ```c++
 int* x = {10, 20, 30};
-int m = 0;
-int W = some_value();
-int U = some_other_value();
+int* m = {0};
+int* W = {0.314};
+int* U = {0.375};
 
 int mem[sizeof(x) / sizeof(x[0]) + 1];
 int o1[sizeof(x) / sizeof(x[0]) + 1];
@@ -131,20 +135,16 @@ int o2[sizeof(x) / sizeof(x[0]) + 1];
 for (int i = 1; i <= sizeof(x)/sizeof(x[0]); ++i) {
   int x = x[i-1];
   if (i == 1) mem[0] = m;
-  int fc_out = W * x;
-  int hidden_out = Y * mem[i-1];
-  int sum = fc_out + hidden_out;
+  int a = W * x;
+  int b = Y * mem[i-1];
+  int s = fc_out + hidden_out;
   int act = sigmoid(sum);
   mem[i] = act;
   o1[i] = act;
   o2[i] = hidden_out;
 }
-
-print_array(o1);
-print_array(o2);
 ```
 
-
 ## Compilation and Execution
 
 Like TensorFlow programs, a PaddlePaddle program is written in Python.  The first part describes a neural network as a protobuf message, and the rest part executes the message for training or inference.
@@ -210,11 +210,11 @@ a = pd.Varaible(shape=[20, 20])
 b = pd.fc(a, params=["fc.w", "fc.b"])
 
 rnn = pd.create_rnn()
-with rnn.stepnet() as net:
-    x = net.set_inputs(a)
+with rnn.stepnet()
+    x = a.as_step_input()
     # reuse fc's parameter
     fc_without_b = pd.get_variable("fc.w")
-    net.set_outputs(fc_without_b)
+    rnn.output(fc_without_b)
 
 out = rnn()
 ```