* Add DataFeeder
A v2 API like data feeder for book demos.
We can feed data directly from reader.
* Fix CI
* Add an unittest for while/rnn op forward
* Add unittest for raw while op backward
* Fix CI
* Use stable_sort in lod_rank_table
It is easy to debug and test when use `stable_sort`and the time
complexity is not changed.
* Add LoDTensorArray
* Stash
* Better debug message for IsInitialized
* Stash
* Better debug message for IsInitialized
* Complete array read/write op unittests