* update 2.0 public api in vision

* fix some flake8 errors

For python, if users write `a, b = var`, the `__getitem__` method will iterate through 0, 1, 2 ... until `__getitem__` throws an IndexError, then stop. The var[0], var[1] will be given to a, b respectively. If more values are given, the unpack size would cause error.

We didn't raise the IndexError in the past and we add statement in `__getitem__` to raise IndexError here to support grammar like `a, b = var` in this PR.

* fix Undefined variables

* fix Undefined variables

* add wget option in download

Change-Id: I4e09e7710f6693bff5388983270781a4ef70519e

* add check nan of inf for dygraph

* add unittest for dygraph

* revert error change

As the title

* support diff dataset tensor place in single process dataloader

* fix unittest failed

* fuse L2Decay and momentum when param.regularizer is set

* add unittest

* refine

* refine _create_regularization_of_grad of momentum

* improve append_optimizer_op

* add paddle.neg api

* add test for neg

* fix an English gammar error in comment

* add lgamma api

* support api paddle.tensor.neg() and paddle.tensor.lgamma()

* modify test_neg_op.py

* update, test=develop

* Support append customize attributes into op_desc in nn.Layer

* fix code style

* support override

* add unittest

* add option verbose for predict api

* add random state generate in DataLoader worker. test=develop

* add parallel_cross_entropy

* add grad for crossentropy

* fix cross entropy

* support state_dict save to memory.

* Perfect unittest

* perfect unittest.

* suport saving binary var to memory

* polish code.

* packag save/load files into pybind/io.py

* polish code .

* add example for save to memory; remove useless save load function(_load_static_dict,_save_dygraph_dict)

* delete _load_static/dygraph_dict;_save_static/dygraph_dict

* edit example of paddle.save/load

* Modify into core.ops.run_program

* add DDout in core.ops.run_program

* fix typo

* add DOut

* fix typo

* put DOut last

* modify API nn.Bilinear's doc, test=develop

* add two attributes for yolo box

* Add wrap for functional api
* Refine the wraped api
* Add unit test for quant functional layers
* Update all unit tests for dygraph qat

* move data to CUDAPlace in advance