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 histc operator, test=develop

* update english doc to 2.0 API, test=develop

* update API from histc to histogram, test=develop
Co-authored-by: Nroot <root@yq01-gpu-255-129-15-00.epc.baidu.com>

* 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

* add trace op, test=develop

* Optimized the implementation of trace op, test=develop

* fix a bug of include in trace_op.h, test=develop

* move trace API from creation to math, test=develop

* modified en doc. test=develop

* add complex trace api

* add complex sum api, test=develop

* modified en doc of complex sum and trace, test=develop

* modified doc and trace API, test=develop

* modified en doc of trace and sum, test=develop

* modified comment in complex kron API, test=develop

* OP Should Not Have Unused Input, test=develop

* add GetExpectedKernelType, test=develop

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

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

* add kron op and its python API, doc and unittests.
* add kron in paddle.complex

* 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

* upload code for tensor.rand

* fix import

* update example, change paddle.tensor.rand to paddle.rand

* change 'variable' to 'Variable',  test=develop

change 'variable' to 'Variable' in description,  test=develop

* add pre-commit check

* add pre-commit check

* pre-commit check, test=develop

* add more unittest code

* trigger ci, test=develop

* pre-commit check, test=develop

* update api and test comment, test=develop

* update api and test comment, test=develop

* add more type check, test=develop

* add detail error info for device, test=develop

* add unnittest, test=develop

* resolve conflict and pre-commit check, test=develop

* add clamp api, test=develop

* add gradient check to reduce ops

* add skip gradient check to reduce ops test=develop

* modify stack api test=develop

* add bmm op and move serval ops from fluid.layers to tensor test=develop

* add addcmul, test=develop

* add paddle.std api test=develop

* update  test=develop

* fix example code format test=develop

* test=develop fix api doc error

add new tensor: max,min,t,eye,log1p; test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* aAdd new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new API : randn

test=develop

* Add new norm api, support frobenius norm and p-order vector norm. test==develop
* combine test files, add more attr checks. test=develop

* add dist op

add addmm op

* add full_like op. test=develop

* add dygraph support. test=develop

* increase coverage. test=develop

* add randint op

* rename op paddle.fluid.matmul to paddle.matmul
* modify original API fluid.layers.matmul,and add new API paddle.matmul

* add index_sample op

* Add the matmul, elementwise_euqal, elementwise_sum ops to API2.0
* Fix the import meesage in common_ops_import
* Update the test cast for mm

* 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 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.

* 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

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

* 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 flip op

* 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