Update VarDesc design

doc/design/var_desc.md

@@ -16,16 +16,23 @@ The computation graph is constructed by Data Node and Operation Node. The concep
 
 ## Definition of VarDesc
 
-A VarDesc should have a name and value, in PaddlePaddle, the value will always be a tensor. Since we use LoDTensor most of the time. We add a LoDTesnorDesc to represent it.
+A VarDesc should have a name, and value. The are two kinds of variable type in compile time, they are `LoDTensor` and `SelectedRows`. 
 
 ```proto
 message VarDesc {
   required string name = 1;
-  optional LoDTensorDesc lod_tensor = 2;
+  enum VarType {
+    LOD_TENSOR = 0;
+    SELECTED_ROWS = 1;
+  }
+  required VarType type = 2;
+  optional LoDTensorDesc lod_desc = 3;
+  optional TensorDesc selected_rows_desc = 4;
+  optional bool persistable = 5 [ default = false ];
 }
 ```
 
-## Definition of LodTensorDesc
+## Definition of TensorDesc
 
 ```proto
 enum DataType {
@@ -38,87 +45,25 @@ enum DataType {
   FP64 = 6;
 }
 
-message LoDTensorDesc {
+message TensorDesc {
   required DataType data_type = 1;
-  repeated int32 dims = 2; // [UNK, 640, 480] is saved as [-1, 640, 480]
-  optional int32 lod_level = 3 [default=0];
+  repeated int64 dims = 2; // [UNK, 640, 480] is saved as [-1, 640, 480]
 }
 ```
 
-## Definition of Variable in Python
-
-In Python API, layer will take Variable as Input, and return Variable as Output. There should be a class `Variable` in python to help create and manage Variable.
-
-```python
-image = Variable(dims=[-1, 640, 480])
-# fc1 and fc2 are both Variable
-fc1 = layer.fc(input=image, output_size=10)
-fc2 = layer.fc(input=fc1, output_size=20)
-```
-### what should class `Variable` Have
-1. `name`.a name of string type is used to mark the value of the Variable.
-1. `initializer`. Since our Tensor does not have value. we will always use some Operator to fullfill it when run. So we should have a initialize method to help add the init operator.
-1. `operator`. Variable should record which operator produce itself. The reaon is:
-  - we use pd.eval(targets=[var1, var2]) to run the related ops to get the value of var1 and var2. var.op is used to trace the dependency of the current variable.
-
-In PaddlePaddle, we use Block to describe Computation Graph, so in the code we will use Block but not Graph.
-
-```python
-import VarDesc
-import LoDTensorDesc
-import framework
-
-def AddInitialOperator(variable, initializer):
-	# add an initialize Operator to block to init this Variable
-
-class Variable(object):
-   def __init__(self, name, dims, type, initializer):
-      self._block = get_default_block()
-      self._name = name
-      self.op = None
-
-      tensor_desc = LoDTensorDesc(data_type=type, dims=dims)
-      _var_desc = VarDesc(name=name, lod_tensor=tensor_desc)
-      self._var = framework.CreateVar(_var_desc)
-      self._block.add_var(self)
+A TensorDesc describes `SelectedRows` and `LoDTensor`. For details of `SelectedRows`, please reference [`SelectedRows`](./selected_rows.md).
 
-      # add initial op according to initializer
-      if initializer is not None:
-          AddInitialOperator(self, initializer)
-
-   def dims(self):
-      return self._var.dims()
-
-   def data_type(self):
-       return self._var.data_type()
+## Definition of LodTensorDesc
 
-   def to_proto(self):
-       pass
+```proto
+message LoDTensorDesc {
+  required TensorDesc tensor = 1;
+  optional int lod_level = 2;
+}
 ```
 
-Then we can use this Variable to create a fc layer in Python.
+A LoDTensorDesc contains a tensor and a lod_level.
 
-```python
-import paddle as pd
-
-def flatten_size(X, num_flatten_dims):
-  prod = 1 # of last num_flatten_dims
-  for i in xrange(num_flatten_dims):
-    prod = prod * X.dims[-i-1]
-  return prod
-
-def layer.fc(X, output_size, num_flatten_dims):
-  W = Variable(pd.random_uniform(), type=FP32, dims=[flatten_size(X, num_flatten_dims), output_size])
-  b = Variable(pd.random_uniform(), type=FP32, dims=[output_size])
-  out = Variable(type=FP32)
-  y = operator.fc(X, W, b, output=out) # fc will put fc op input into out
-  pd.InferShape(y)
-  return out
-
-x = Variable(dims=[-1, 640, 480])
-y = layer.fc(x, output_size=100)
-z = layer.fc(y, output_size=200)
+## Definition of Variable in Python
 
-paddle.eval(targets=[z], ...)
-print(z)
-```
+For Variable in Python, please reference [`Python API`](./python_api.md).