* make Numpy version is below 1.19.3

* fix 2.0 doc

* change paddle.fluid.layers.reduce_sum to paddle.sum in sample codes

* format codes

* add tensor clone

* fix unittest test_var_base

* modify cond while_loop to paddle.static.nn.cond

* modify crop_tensor to paddle.crop

* modify Variable to paddle.static.Variable

* remove nn.beam_search, nn.beam_search_decode, nn.gather_tree

* remove bpr_loss, center_loss, rank_loss, smooth_l1, teacher_student_sigmoid_loss, edit_distance, sampled_softmax_with_cross_entropy in nn.functional

* remove apis in nn.functional.learn_rate.py

* remove pool2d, pool3d, adaptive_pool2d, adaptive_pool3d in nn.functional

* remove apis in nn.functional.vision

* remove erf, soft_relu in nn.functional.activation

* remove apis in nn.functional.extension

* remove nn.functional.rnn

* remove hash from nn.functional.lod

* remove row_conv from nn.functional.extension

* remove one_hot, pad2d, pad_constant_like from nn.functional.common

* remove nn.gather_tree, nn.BilinearTensorProduct, nn.Pool2D, nn.Pad2D

* remove apis from optimizer.__init

* remove tensor.creation.fill_constant

* remove elementwise_mul in nn.functional.common and  modify to paddle.multiply

* remove  tensor.stat.reduce_mean

* remove reduce_all, reduce_any in tensor.logic

* remove apis in tensor.math

* remove apis in tensor.__init__

* remove has_inf, has_nan in tensor.search

* remove apis in framework.__init__

* remove apis in paddle.__init__

* remove apis in nn.functional.__init__

* modify removed alias apis to raw api in doc and unittests

* fix remove grid_sample bug

* modify removed alias apis to raw api in doc and unittests

* modify removed alias apis to raw api in doc and unittests

* modify removed alias apis to raw api in doc and unittests

* modify removed alias apis to raw api in doc and unittests

* modify removed alias apis to raw api in doc and unittests

* modify removed alias apis to raw api in doc and unittests

* delete alias api relastions in doc

* reserve paddle.compat, paddle.sysconfig

* remove unittest for paddle.reduce_all, paddle.reduce_any

* modify removed alias apis to raw api in doc and unittests

* recover paddle.save and paddle.load

* resolve conflicts

* fix sample code missing paddle.enable_static() bug

* fix sample code missing paddle.enable_static() bug

* fix to_string sample code error

* add sumary feature

* refine printting tensor

* add sci_mode

* add sample code

* fix indent error

* fix _format_item

* polish code

* support item indent

* add ut

* set place for ut

* fix py2 issue

* fix ut

* Support assignment to a Variable in dynamic mode. Note: not deal with backward.

* Rewrite VarBase __setitem__ for high-performance.

* try to test 3 means to do __setitem__ and test the performance of 3 means.

* Retain the means of the highest performance: C++ code and don't trace op.

* Makes the Bind Method more intelligent

* Makes the Bind Method more intelligent

* fix unittest

* fix unittest

* fix conflict

* update the demo code and the doc of varbase.backward.

* update the doc of the fake interface `paddle.fluid.Variable`.

* remove BackwardStrategy.

* support Baidu AI Accelerator
  * test=kunlun

* minor
 * test=kunlun

* support xpu op in separate file
 * test=kunlun

* update XPU error message and remove duplicated code

 * test=kunlun

* minor
 * test=kunlun

* minor
 * test=kunlun

* expose and unify the Tensor concepts to the user

* expose tensor to user

* add copy place for Tensor

* add copy place for Tensor

* add note

* add macro PADDLE_WITH_CUDA

* remove RUN_TYPE=DIST

* fix some error

* add auto_cast, test=develop

* add loss scaler, test=develop

* add comments, test=develop

* refine code, test=develop

* refine code, test=develop

* do not set flags automatically, test=develop

* fix custom op bug, test=develop

* add more test, test=develop

* refine enable logic, test=develop

* enable amp test with GPU, test=develop

* add unittest

* add test for found_inf

* follow comments

* follow comments

* remove global variable, use singleton

* add some notes

* update comments

* update comments

* update comments

* add use_dynamic_loss_scaling argument

* refine found_inf

* refine found_inf

* fix the double grad bug for the star gan. test=develop

* update the retain_graph parameter doc. test=develop

* add the unit test for the retain_graph parameter. test=develop

* Modify tmp var name prefix in dygraph test=develop

* refine comment test=develop

* support selectedrows allreduce in multi-cards dygraph, test=develop

* remove useless import modules in unittests, test=develop

* add nccl cmake to get nccl version, test=develop

* add if-condition to compiled correctly, test=develop

* add detail version parseing for old nccl, test=develop

* polish camke details, test=develop

* fix remove test cmake error, test=develop

* fix cmake condition, test=develop

* change unittest camke list, test=develop

* fix unittest cmake rule, test=develop, test=framep0

* fix bug of varbase.__getitem__, test=develop

* fix bug of float and other type, test=develop

* add new macro BOOST_GET_SAFELY & unittests, test=develop

* add different macro type, test=develop

* fix get macro type in executor, test=develop

* four macro part change backup

* using one macro for all case, test=develop

* revert attribute change, test=develop

* change to three func to solve gcc4.8 bug, test=develop

* polish some details, test=develop

* raise index error when slice out of range; test=develop

* add uni test; test=develop

* fix format error; test=develop

* add comment for py::index_error; test=develop

* polish error message; test=develop

* polish error message; test=develop

* update name generator, test=develop

* use c++ unique name generator, test=develop

* add DataLoader, Dataset, BatchSampler

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

* enhance error msg related Tracer, test=develop

* refine err msg, test=develop

* follow comments, test=develop

* static model runner basic implement, test=develop

* add run program op to execute loaded program, test=develop

* refactor static model runner & run program op, test=develop

* reset engine.cc to resolve conflict

* adapt the change of dygraph double grad, test=develop

* refactor impl to solve control flow error, test=develop

* clear debug code, test=develop

* fix ci str compatible error & checkout dygraph grad maker & add example, test=develop

* hide api & add op test, test=develop

* fix run program op test places error, test=develop

* fix program by review comment, test=develop

* delete change var desc name, test=develop

* fix other program by review comment, test=develop

* remove _static_graph_guard, test=develop

* add selectedrows test, test=develop

* remove desc parser, test=develop

* fix detail program, test=develop

* change socpe create & add test, test=develop

test=develop

* expose dygraph.grad api, test=develop, test=document_fix

* add more parameter in dygraph.grad API, test=develop

* add only_inputs=True parameter, test=develop

* follow comments, test=develop, test=document_fix

* fix typo, test=develop, test=document_fix

* add double grad implementation for dygraph, test=develop

* polish code, add uts, test=develop

* fix place bug, test=develop

* polish codes, add more uts for coverages, test=develop

* add no_grad_set, test=develop

* add star gan ut, test=develop

* follow comments, test=develop

* move __getitem__ to cpp

* bug fix

* add type check and gil release

* support negative step with omitted ends
test=develop

* code refine
test=develop

* bug fix
test=develop

* slice always return different pyobj
test=develop

* add lodtensor share memory & serialization, test=develop

* fix windows compile error, test=develop

* deal vartype pickle & fix unittest matching error message, test=develop

* update timeout variable name, test=develop

* refactor memory map implement, test=develop

* clear mmap file discripter when exit unexpectedly, test=develop

* remove the child process fd in advance, test=develop

* remove mmap fds after Queue.put in child process, test=develop

* add hard unittests for register exit func, test=develop

* fix python2 compatibility problem in unittest, test=develop

* fix exception unittest error, test=develop

* polish code based review comment, test=develop

cmake选项中添加了WITH_NCCL，显示指定是否编译NCCL的部分代码，WITH_NCCL默认打开，但如果WITH_GPU为OFF，则关闭WITH_NCCL

添加了PADDLE_WITH_NCCL定义

单机单卡能够关闭NCCL编译，多卡的话需要默认打开NCCL，如果关闭NCCL，则只能使用单卡
Co-authored-by: N石晓伟 <39303645+Shixiaowei02@users.noreply.github.com>

remove unnecessary template.

* add multiprocess for dygraph data loader, test=develop

* polish code & add safe gurad, test=develop

* refactor dygraph dataloader & add signal handler, test=develop

* fix member initializer compile error on ci, test=develop

* fix member initializer compile error one more, test=develop

* remove useless config, test=develop

* skip windows incompatible problem, test=develop

* add unittest for coverage, test=coverage

* add more exception unittest case, test=develop

* deal with signal handler coverage, test=develop

* polish code & add signal handler tests, test=develop

* deal with coverage ci problem, test=develop

* split data loader test & coverage ci fix, test=develop

* remove test_imperative_data_loader_with_exception, test=develop

* remove singal process except test case, test=develop

* add exception tests again & remove sample list test, test=develop

* split normal and exception unittests to diff class, test=develop

* polish doc for use_multiprocess effect in static mode, test=develop

* modify fc to linear in sample code, test=develop

* remove FC, test=develop

* remove warnings, test=develop

* drop fluid/imperative/README.md , test=develop

* change fc to linear, test=develop

* polish code style, test=develop

* add op function generator, test=develop

* add unittest, test=develop

* follow comments, test=develop

* fix windows compilation problem, test=develop

* refine init function, test=develop

* add tests, test=develop

* remove extern, which may cause symbol error in gcc-4.8, test=develop

* test=develop, fix docker with paddle nccl problem

* don't expose numerous Tensor.set(), test=develop

* fix condition, test=develop

* fix float16 bug, test=develop

* feed should be Tensor or np.array, not Variable or number, test=develop

* use forcecast to copy numpy slice to new array, test=develop

* remove float16-uint16 hacking, test=develop

* add variable method to varbase and refactor to_variable to support return varbase

* support kwargs in varbase constructor

* add VarBase constructor to support default python args

* refine varbase initial method

* reset branch

* fix ut for change VarBase error info to PaddleEnforce

* cherry is parameter change before

* overload isinstance to replace too many change of is_variable

* rm useless files

* rm useless code merged by git

* test=develop, fix some ut failed error

* test=develop, fix test_graph_wrapper

* add some tests, test=develop

* refine __getitem__, test=develop

* add tests, test=develop

* fix err_msg, test=develop

* support SelectedRows in dygraph, test=develop

* fix bug of _grad_ivar interface, test=develop

* add optest for support seletedrows, test=develop

* fix bug for gradient_accumulator in GPU mode, test=develop

* fix error when Selectedrows addto LodTensor in sorted_gradient mdoe in dygraph, test=develop

* refine and simplify gradient accumulator code, test=develop

* add optest, test=develop

* add optest and simplify code, test=develop

* fix bug for test_imperative_selected_rows, test=develop

* add optest for Coverage, test=develop

* fix gradient interface and simplify code, test=develop

* update api for gradient, test=develop

* fix ShareDim's bug in DygraphExecutionContext class, test=develop

* add optest, test=develop

* polish jit trace codes, test=develop

* polish codes again by removing var_id, test=develop