Created by: wangkuiyi

In paddle/fluid/memory/detail/, the three modules

• memory_block.{h,cc}
• meta_data.{h,cc}
• meta_cache.{h,cc}

are highly correlated:

1. memory_block.h depends on meta_cache.h

   https://github.com/PaddlePaddle/Paddle/blob/bcb46f554986a07330afc2bbf34356322d4b833d/paddle/fluid/memory/detail/memory_block.h#L36-L45

2. meta_cache.h depends on meta_data.h

   https://github.com/PaddlePaddle/Paddle/blob/bcb46f554986a07330afc2bbf34356322d4b833d/paddle/fluid/memory/detail/meta_cache.h#L40

3. meta_data.h depends on memory_block.h

   https://github.com/PaddlePaddle/Paddle/blob/bcb46f554986a07330afc2bbf34356322d4b833d/paddle/fluid/memory/detail/meta_data.h#L46-L47

Should we merge these three modules into one, or at least change the following CMakeLists.txt rules:

https://github.com/PaddlePaddle/Paddle/blob/bcb46f554986a07330afc2bbf34356322d4b833d/paddle/fluid/memory/detail/CMakeLists.txt#L9-L13

into one

cc_library(meta_data_cache SRCS meta_data.cc meta_cache.cc memory_block.cc)