* Unify fluid submodules to fluid module

Change books just use `import fluid`, not submodules

* Remove g_main_program/g_startup_program

Use default_main_program/default_startup_program instead

* Typo

* Fix CI

* "add asnumpy interface"

* Just for unittest

* Change unittests for numpy I/O

* Fix CI

* Change all `data_type` in Python to `dtype`

* Change `date_type` in C++ to `dtype`

* Refine

* init commit

* change some dir name

Follow comments

Used for shrink memories state in DyRNN. The height of state could
be shrinked after running a step block.