* refine unsqueeze, test=develop

* update unsqueeze, test=develop

* refine unsqueeze, test=develop

* refine unsqueeze, test=develop

* update

* remove None, test=develop

* follow comments

* support bool

* update doc

* follow comments

* merge develop

* refine stack in dygraph mode, test=develop

* update stack, test=develop

* rename Variable to Tensor, test=develop

* add name args to fluid.layers.stack, test=develop

* follow comments

* update doc

* update doc

* follow comments

* refine squeeze, test=develop

* update squeeze, test=develop

* refine compile-time infershape, test=develop

* add more unittest, test=develop

* follow comments, test=develop

* add update_api, test=develop

* follow comments, test=develop

According to paddle 2.0 standard
1, change flip api attr name 'dim' to 'axis'.
2, support empty axis
3, change example code to imperative mode.

* add alias in paddle.nn and paddle.tensor test=develop

* add alias in paddle.nn and paddle.tensor dir test=develop

* fix same conflict manually test=develop

* update fc and dygraph alias test=develop

* fix initalizer.py typo test=develop

* add unbind pyhon api. test=develp

* define fluid alias in paddle.tensor and paddle.nn dir test=develop

* from layer import loss in paddle.nn dir test=develop

* add gather split squeeze stack unsqueeze api test=develop

* add gather split squeeze stack unsqueeze api test=develop

* fix bug test=develop

* fix bug test=develop

* fix bug test=develop

* fix bug test=develop

* fix bug test=develop

* add flip op