* Add the sum op to API 2.0, test=develop
* Fix the import meesage in common_ops_import

*  add new op paddle.tensor.mul(x, y, x_num_col_dims=1, y_num_col_dims=1, out=None, name=None)
* add new op paddle.tensor.pow(input, exponent, out=None, name=None)
* add the aboved two new ops unittest (test api param out and name)

add tril op and  triu op

* add unbind op
unbind(tensor, dim=0):
说明：移除指定维后，返回一组数组，包含了沿着指定维切片后的各个切片。

tensor(Tensor) -- 输入Tensor
dim(int) -- 删除的维度

示例：
 Input = [[1,2],
           [3,4],
           [5,6]]
  axis = 0
  Output[0] = [1,2]
  Output[1] = [3,4]
  Output[2] = [5,6]

* add new op paddle.tensor.div(x, y, out=None, name=None) 
* add gpu and dygraph unittests.
* Performance optimization: scale op is not called when alpha=1. 
* op error message optimization.

This CR makes two changes:

1. In old loop_transformer, if a class variable, such as "self.a, foo.bar" is a loop var, the Dy2stat will fail because `def func(self.foo)` is not legal syntax. We support class variable by renaming.

2. After https://github.com/PaddlePaddle/Paddle/pull/22892 is merged, we can support `while x < 10` in dygraph. I enable those tests in corresponding Dy2stat

* solve the conflict of ops with the same name. test=develop

* Add the argmax op to API 2.0, test=develop

* Fix the compiler problem in arg_max op, test=develop

* Fix the import meesage in common_ops_import, test=develop

* Fix the default dtype of arg_min_max, test=develop

* fix init_gflags with 'python -c', test=develop

* add test, test=develop

* use sys.executable instead of python, test=develop

* keep dummy, test=develop

* add a new op paddle.tensor.sort(), test=develop
* add a new op paddle.sort(), test=develop

Update the new api ops of creation ops to the api 2.0

* Add allclose Op, and its function is analogous to numpy.allclose. It returns True if two tensors are elementwise equal within a tolerance.

* sin sqrt tanh atan add out, test=develop

* optimize doc, test=develop

* add dygraph test, test=develop

* add tdm child op

* Add unitTest for `Tensor==constant` for ifElse in dygraph2static test=develop

* add flip op

* add functional conv

* add test and doc for function convs, test=develop

* update ConvTransposeOp's InferShape and error message, test=develop

* fix gen_worker_desc in hogwild, it may cause error in transpile mode
* test=develop

* update linspace, equal operators to API 2.0, test=develop

* equal support higher performance CUDA kernel, test=develop

* update comment of equal&linspace operator, test=develop

* update comment of equal&linspace operator, test=develop

* support if logic for Variable in dygraph, test=develop

* fix test_learning_rate_scheduler.py, test=develop

* fix optest, test=develop

* fix error message, test=develop

* fix optest,test=develop

* fix comment, test=develop

* support forward hook for dygraph, test=develop

* add optest for forward_hook in dygraph, test=develop

* add optest, test=develop

* polish code, test=develop

* add sample code, test=develop

* rename forwrd_hook to forward_post_hook, test=develop

* fix the api description, test=develop

* fix api description, test=develop

* prune train program by fetch_list, test=develop

* add unittest for prune, test=develop

* fix pruned feed, test=develop

* support ParallelExecutor and feed prune, test=develop

* add comments, test=develop

* update unittest, test=develop

* update unittests, test=develop

* remove debug code, test=develop

* support cond in clone, test=develop

* support cond in prune, test=develop

* support multiple minimize, test=develop

* support cache, test=develop

* fix _copy_param_info_from, test=develop

* support python2 str, test=develop

* remove debug code, test=develop

* fix bug of caching CompiledProgram, test=develop

* fix multi_device issue, test=develop

* tmp

* support tuple in fetch_list and overriding use_prune, test=develop

* dont use nonlocal in python2, test=develop

* remove nonlocal, test=develop

* code clean, test=develop

* code clean, test=develop

* feed list, test=develop

* test adam, test=develop

* follow comments, test=develop

* reduce duplicate code, test=develop

* update comments, test=develop

* Add unittest for transformer prediction in dygraph_to_static.

* fix bug in fill_constant api. 

* Make transpose support size 0. test=develop

* support tensor array create read write in dygraph
test=develop

* minor fix
test=develop

* support tensor_array_to_tensor
test=develop

* add while loop case and err msg
test=develop

* refine ut
test=develop

* add L1Loss

* support L1Loss, test=develop

* add test, test=develop

* fix batch, test=develop

* follow comments, test=develop

* fix stat var in hogwild worker
* test=develop