* first commit
* Python API for while op
* Python Unittest for simple while_op forward
* fix out to be list
* Fix UT
* VarType
* Fix several bugs
* Fix bug
* Fix bug
* Fix Bug
* Fix bug
* Fix unittest
* Remove debug log
* Add comments
* add PADDLE_ENFORCE
* while_grad_op first commit
* Add `BlockDescBind::FindRecursiveOrCreateVar()` and fix bugs
* not sure how to setdim of while outputs
* push for test
* add executor vlog
* fix bug of while_op cond
* Several enhancement for code
1. Backward always infer shape & infer var type. Since there are RENAME
variables will be created when creating backward operator, but their
shape & var types are not inferenced.
2. Never use SomePtr-> directly, since every pointer could be nullptr if
it is a function return value. Add `detail::Ref` to cast pointer to
reference safely.
3. Enhance error message for backward.
4. Infer data type of variable in `sum` and `tensor_write`
* Fix bugs of while_op gradient
* Fix several bugs of while_op grad
* fix fill zeros like
* fix 3 >= 3
* fix place holder shouldn't be null
* fail on sum op
* Fix SumOp of TensorList
* clean up
* pass while test
* fix test_array_write_read
* pass sum op
* Support int/int64 for fill_constant_batch_size_like
* Fix compile
Yang Yang(Tony)