Add Skeleton of Python API design

doc/design/python_api.md

+# Design Doc: Python API
+
+<!-- 引言，说明问题 -->
+
+The top level user API in Python should be as same as API in `paddle.v2` after refactoring Paddle from a layer based framework to an operator based framework. There are many new classes in CPP in [compile time] for describing neural networks, such as `Variable`, `Operator`, `Block`. The issue about current design is how to give a proper way to wrap the C++ API to `paddle.v2` API and writing layers in Python.
+
+<!-- 说明为什么我们要先用runtime概念实现Python API。说明我们必须要同时考虑编译器API，进而让之后迁移更简单 -->
+
+This implementation of Python API includes two steps.
+
+1. Implement the Python API using current C++ runtime concepts.
+2. Replace the implementation by using compile-time concepts when they are completed.
+
+...
+
+
+## Python Class about compile-time concepts
+
+<!-- 引言，引出这个表格 -->
+| Python Class | Compile-time protobuf |
+| --- | --- |
+| Block | BlockDesc |
+| Operator | OpDesc |
+| Variable | VarDesc |
+
+
+### Block
+
+<!-- TODO -->
+
+```python
+class Block(objects):
+	def __init__(self, parent=None):
+		self.vars_ = map<string, Variable>()
+		self.ops_ = vector<Operator>()
+		if parent is None:
+			self.global_vars = map<string, Variable>()
+			self.parent=None
+		else:
+			self.parent = parent
+			self.global_vars = None
+	
+	def create_global_vars(...):
+		if self.parent is not None:
+			return self.parent.create_global_vars(...)
+		else:
+			return self.global_vars.new()
+```
+
+
+### Operator
+
+<!-- TODO -->
+
+```python
+class Operator(object):
+	def __init__(self, type, inputs, outputs, attrs):
+		# create OpDesc in Python
+		op_desc = ...
+		self.cpp_op_desc_ptr = cpp.to_cpp_op_desc(op_desc)
+		cpp.infer_shapes(self.cpp_op_desc_ptr, inputs, outputs)
+		outputs.op = self
+
+	def type(self):
+		return self.cpp_op_desc_ptr.type()
+```
+
+### Variable
+
+<!-- TODO -->
+
+```python
+class Variable(object):
+    def __init__(self, shape, dtype="float32", name=None, block=None):
+        if name is None:
+            if prefix is not None:
+                name = unique_name_generator(prefix)
+            else:
+                name = unique_name_generator("unknown")
+        self.name = name
+        self.block = block
+        self.cpp_var_desc_ptr = ...
+        self.op = None
+
+    def shape(self):
+        cpp_shape = self.cpp_var_desc_ptr.shape()
+        return [None if elem < 0 else elem for elem in cpp_shape]
+```
+
+### Parameter
+
+<!-- 虽然Parameter不是编译器的概念，但是Python维护一个Parameter可以帮助我们构造计算图，知道哪个参数是可更新的等等 -->
+
+<!-- 参数 is a special Variable -->
+
+```python
+class Parameter(Variable):
+    def __init__(self, trainable, initialize_attrs, optimize_attrs):
+        pass
+```
+
+## Layer Functions
+
+<!-- 给出一个Demo如何写Data Layer和FC Layer -->