* 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

* Add API for switch default program

* Two functions: switch_main_program/switch_startup_program
* A guard: program_guard. Users can use the `with` statement change
  default programs
* Change unittests in `test_layers`

* Fix CI

* Fix CI

* Fix CI

Fix #6087

* fix python gflags init

* format code

Revise comments in rank_loss_op

* add back print_operators_doc

* fix style check

* fix style check

Amend license and comments in reshape_op

Refine doc for smooth l1 loss op.

* Add survey of support of half in different CUDA versions

* small fix

My unpool max 2d

fix conv and conv_trans op doc

* Make param_attr as a strong typed class

Fix #5819

Small fix in profiler.py of fluid.

Fix gru_op related code style

* fix compile on cudnn7

* update

* update

* make silent

* refine install docs

* do not remove files

* follow comments

* update

Add CUDA profiler tools in new framework.

Update the content of design doc for distributed architecture