Move Fluid C++ code from /paddle to /paddle/fluid

[wangkuiyi]

Fixes #8345

[wangkuiyi]

The CI failed at the stage of building the wheel package:

[01:41:10][Step 1/1] error: package directory '/paddle/build/paddle/framework' does not exist
[01:41:10][Step 1/1] make[2]: *** [python/build/.timestamp] Error 1

I have no clue with the process that builds the wheel package. Could @Yancey1989 @typhoonzero please help me with this? Thanks!

[typhoonzero]

@wangkuiyi should change this line: https://github.com/wangkuiyi/Paddle/blob/154368b1a0f81dda8393a9aa8687d7da44b3bead/python/setup.py.in#L112

[luotao1]

In order to make paddle_docs sucussfully, should change:
https://github.com/wangkuiyi/Paddle/blob/a6c1bffa471798fa36829c8cb24c9bd993bd2baf/paddle/fluid/operators/CMakeLists.txt#L4
from set(pybind_file ${PADDLE_SOURCE_DIR}/paddle/pybind/pybind.h)
to set(pybind_file ${PADDLE_SOURCE_DIR}/paddle/fluid/pybind/pybind.h)

And change the file in .gitignore: https://github.com/PaddlePaddle/Paddle/blob/develop/.gitignore#L36
from paddle/pybind/pybind.h
to paddle/fluid/pybind/pybind.h

[luotao1]

paddle/string should be moved to paddle/fluid/string as well (TODO in next PR)