* add memopt level

* add opt level for image classification demo

* clean code

* add delete op

* clean code

* test machine translation demo

* clean code

* clean code

* skip fill constant with force cpu

* clean code

* clean code

* refine code

* clean code

* fix bug

* change learning_rate_decay to learning_rate_scheduler
* optimize code
* change nn.cast to tensor.cast

* Add reset and guard to unique_name

* Adding Python boilerplate code for Go op

* Add very basic test case

* Adding the python logic for go routine

* Fix syntax

* Changing test to notest

* Rename Routine to Go

* Combining GoGuard and Go in one class

* Modify test

* Adding fluid close channel

* Fixing __init__.py for calling fluid.go()

* Adding stubs for channel methods and updating test case

* Removing import *

* Adding imports from concurrency

* add basic interface for learning rate decay
* add exponential_decay
* add natural_exp_decay
* add inverse_time_decay

* "unify flags"

* "fix init"

The English of the previous API is bad.

* add memory optimization transpiler demo

* add memory benchmark compile option

* add gflags instead of macro

* refine code

* "fix decode bug"

* "follow commnet"

* "fix error"

* "fix hook bug"

* fix based comment

* fix copyright

* fix based on comment

Users can use `a+b`, `a*10`.

* add copyright hook

* add copyright hook

* refine copyright hook

* "test copyright hook"

* fix check style

* fix ci

* "init use all default devices"

* "fix init test"

* "change operator interface"

* "move devicepool to device_context"

* "fix operator test"

* "fix op_registry Run interface"

* "net op passed. Need to fix nccl multi-Context"

* "add nccl group function"

* "add nccl group function"

* "fix gpu count exceed 32 error"

* "fix recurrent op, nccl op"

* "change the other operators interface with Place"

* "fix typo"

* "fix pybind"

* "fix device in python side"

* "fix pybind failed"

* "add init for test"

* "fix CI"

* Support Clip in param_attr

* Fix the order of clip & regular

Regular is not need to be clipped

* Add DataFeeder

A v2 API like data feeder for book demos.
We can feed data directly from reader.

* Fix CI

* Remove batch_size_dim for feeder

Also add __all__ to data_feeder.py

* Follow comment

* fix python gflags init

* format code

* Make param_attr as a strong typed class

Fix #5819

Change books just use `import fluid`, not submodules

* init commit

* change some dir name

* add init_gflags interface

* refine code

* follow comments