Created by: wangkuiyi

1. There are many targets in paddle/fluid/memory/CMakeLists.txt, but their dependencies are not clearly expressed: https://github.com/PaddlePaddle/Paddle/blob/535646cf256cef422f34ebe19a7a1e311e4225d6/paddle/fluid/memory/CMakeLists.txt#L3-L14

2. The target memory should be malloc; the target paddle_memory should be memory and the only target exposed by the C++ package paddle/fluid/memory.

3. The dependencies inside the memory package should be

   memory -> malloc -> (detail/)buddy_allocator -> (detail/)system_allocator
         \-> memcpy                            \-> (detail/)memory_block

   where malloc is what currently known as memory, and memory is what currently known as paddle_memory.