Init Python API

Following the design
* https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/python_api.md

Just written `Program`, `Block` and unittest of program.

python/paddle/v2/framework/graph.py

+import paddle.v2.framework.core as core
+
+
+class Block(object):
+    def __init__(self, program, idx):
+        self.proto = program.proto.block(idx)
+        self.vars = dict()  # var_name --> var
+        self.ops = list()  # operator list
+        self.program = program
+
+    @property
+    def parent_idx(self):
+        return self.proto.parent
+
+    @property
+    def idx(self):
+        return self.proto.id
+
+
+class Program(object):
+    def __init__(self):
+        self.proto = core.ProgramDesc.instance()
+        assert self.proto.num_blocks() == 1
+        self.blocks = [Block(self, 0)]
+        self.current_block_idx = 0
+
+    def global_block(self):
+        return self.blocks[0]
+
+    def current_block(self):
+        return self.blocks[self.current_block_idx]
+
+    def create_block(self):
+        new_block_idx = len(self.blocks)
+        self.proto.append_block(self.current_block().proto)
+        self.current_block_idx = new_block_idx
+        self.blocks.append(Block(self, self.current_block_idx))
+        return self.current_block()
+
+    def rollback(self):
+        self.current_block_idx = self.current_block().parent_idx
+
+
+# program is a global instance.
+g_program = Program()

python/paddle/v2/framework/tests/test_program.py

+import unittest
+from paddle.v2.framework.graph import g_program
+
+
+class TestProgram(unittest.TestCase):
+    def test_program(self):
+        b = g_program.current_block()
+        self.assertEqual(-1, b.parent_idx)
+        self.assertEqual(0, b.idx)
+
+        b = g_program.create_block()
+        self.assertEqual(1, b.idx)
+        self.assertEqual(0, b.parent_idx)
+
+        b = g_program.create_block()
+        self.assertEqual(2, b.idx)
+        self.assertEqual(1, b.parent_idx)
+
+        g_program.rollback()
+
+        b = g_program.current_block()
+        self.assertEqual(1, b.idx)
+        self.assertEqual(0, b.parent_idx)
+
+        b = g_program.create_block()
+        self.assertEqual(3, b.idx)
+        self.assertEqual(1, b.parent_idx)
+
+        g_program.rollback()
+        b = g_program.current_block()
+        self.assertEqual(1, b.idx)
+        self.assertEqual(0, b.parent_idx)
+
+
+if __name__ == '__main__':
+    unittest.main()