add imperative mode design

test=develop

paddle/fluid/imperative/README.md

+# Overview
+
+Imperative Programming
+
+# Related Works
+
+## Pytorch
+https://pytorch.org/
+
+## TensorFlow Eager
+https://www.tensorflow.org/guide/eager
+
+# Design
+
+## API
+```python
+class Layer(object):
+
+  def __call__(inputs):
+    # build some parameter once.
+    # ...
+    return self.apply(inputs):
+
+
+  def apply(inputs):
+    # forward logic with paddle operators. backward auto-generated.
+
+
+class PyLayer(core.PyLayer):
+
+  def __call__(cls, inputs):
+    # trace the logic.
+
+  @staticmethod
+  def forward(inputs):
+    # any forward logic implemented with numpy io.
+
+  @static method
+    # any backward logic implemented with numpy io.
+```
+
+
+## Tracer
+
+Python Variable -> C++ VarBase -> C++ Variable -> C++ Tensor
+
+
+```cpp
+class Tracer {
+ public:
+  explicit Tracer(framework::BlockDesc* root_block) : root_block_(root_block) {}
+
+  virtual ~Tracer() {}
+
+  void Trace(OpBase* op,
+             const std::map<std::string, std::vector<VarBase*>>& inputs,
+             const std::map<std::string, std::vector<VarBase*>>& outputs,
+             framework::BlockDesc* block, const bool stop_gradient = false);
+
+  std::vector<VarBase*> PyTrace(OpBase* op, const std::vector<VarBase*>& inputs,
+                                bool stop_gradient = false);
+};
+```
+
+## Autodiff
+
+Lots of research already.
+https://autodiff-workshop.github.io/
+
+
+## Tests
+
+* All op tests run once in static graph, once in imperative mode.
+
+## Refactor
+
+* All function layers with parameters converted to class Layers.
+* Models converted to imperative mode.
+
+
+# Examples
+
+```python
+class MyLayer(fluid.imperative.Layer):
+    def __init__(self):
+        super(MyLayer, self).__init__()
+
+    def forward(self, inputs):
+        x = fluid.layers.relu(inputs)
+        x = fluid.layers.elementwise_mul(x, x)
+        x = fluid.layers.reduce_sum(x)
+        return [x]
+
+
+class MyPyLayer(fluid.imperative.PyLayer):
+    def __init__(self):
+        super(MyPyLayer, self).__init__()
+
+    @staticmethod
+    def forward(inputs):
+        return np.tanh(inputs[0])
+
+    @staticmethod
+    def backward(inputs):
+        return np.array(dout) * (1 - np.square(np.array(out)))
+
+
+class MLP(fluid.imperative.Layer):
+    def __init__(self):
+        super(MLP, self).__init__()
+        self._fc1 = FC(3,
+                       fluid.ParamAttr(
+                           initializer=fluid.initializer.Constant(value=0.1)))
+        self._fc2 = FC(4,
+                       fluid.ParamAttr(
+                           initializer=fluid.initializer.Constant(value=0.1)))
+
+    def forward(self, inputs):
+        x = self._fc1(inputs)
+        x = self._fc2(x)
+        x = fluid.layers.reduce_sum(x)
+        return x
+
+
+ np_inp = np.array([[1.0, 2.0], [3.0, 4.0]], dtype=np.float32)
+ with fluid.imperative.guard():
+     var_inp = fluid.imperative.base.to_variable(np_inp)
+     mlp = MLP()
+     out = mlp(var_inp)
+     dy_out = out._numpy()
+     out._backward()
+```
+
+# Plan
+
+2.1，3 fulltime, Can run a few simple models. (Currently, 2 20% engs)
+
+4.1, 4 fulltime, Can run 6 models, Performance 70% Pytorch. Release alpha.
+
+6.1, 5 fulltime, Performance close to Pytorch, can run multi-devices. Release Beta.
+
+8.1, 5 fulltime, Works in general. Covert current models to use imperative mode.
+
+12.1, 5 fulltime, Can compile to static graph, support more optimizations.
+
+# Discussion
+
+TODO.