* sequnce mask support max length tensor input; test=develop

* add rnn_impl.py; test=develop

* add basic gru lstm unittest; test=develop

* fix api spec; test=develop

* fix sequence_mask op bug;
test=develop
test=document_preview

* change +-*x to elmentwise_op; test=develop

* add mkl flag; test=develop

* fix rnn impl bug; test=develop

* update api spec; test=develop

* fix doc bug; test=develop

* fix lstm bugs; test=develop

test=release/1.5

* Fix the GetExpectedKernelType of add_position_encoding_op.

* Fix the doc of lstm_unit outputs in nn.py.

test=release/1.5

* Use TensorCopySync for sequence_unpad op

* Fix the tensor memory alloc bug

test=release/1.5

* fix slice op bug; test=develop

* fix variabel test bug; test=develop

* remove slice while true; test=develop

Repair error prompt: Users are prompted to check whether the model or parameter files are damaged when loading parameters are wrong.

* cherry pick 18000, test=release/1.5

* Update backward appending stragety to support double backward and fix some bug. (#18104)

* Update backward.py:
     - If there is no input grad var in all outputs of previous ops, do not append this op into graph.
     - Only apply this stragety when double backward.
* Update some double backward op.
* Update sum_op to judge whether a tensor is empty by numel or IsInitialized().

* fix save/load in fleet (#17675)

* fix save/load in Fleet
* add UT framework of Fleet (#18058)

* add paddle cloud role maker for customized usage, note this is only for industrial users that have cloud environment pre-configuration (#18121)

add paddle cloud role maker for specific cloud usage. This pr will simplifies user's configuration in distributed training.

* assign role_maker before use (#18137)

Cherry pick retinanet_target_assign_op(#17893), sigmoid_focal_loss_op(#17895) and retinanet_detection_output_op(#17896) for supporting retinanet (#18141)

* test=release/1.5
Fix conflicts in test_layers.py when adding target assign operator for supporting retinanet. Cherry pick #17893

* test=release/1.5
Add sigmoid focal loss operator for supporting retinanet. Cherry pick #17895

* test=release/1.5
Add detection output operator for supporting retinanet. Cherry pick #17896

* test=release/1.5
fix wrong code style in test_layers.py when cherry pick retinanet_target_assign #17893

* test=release/1.5
Fix type error of std::pow in sigmoid_focal_loss. Cherry pick #17895

Add cascade rcnn support.

cherry pick for deform roi pooling

* fix py_reader iterable bug, test=release/1.5

* move data from buffered_reader,test=release/1.5

* update CPU_NUM config
test=develop

test=release/1.5
cherry-pick from #17952
The scatter op has a calc bug when the indices has same index, the scatter op use overwrite mode to calculate the same index, fix this bug by using the accumulate mode to calculate the same index.At the same time, the gather op has the same bug when the op calc the grad. And we use the lib of open-blas and eigen to optimize the time cost in accumulate mode.

Added unit test for QAT FP32 & INT8 comparison (#17814)
Disable MKLDNN FC in Resnet50 test (#18030)

test=release/1.5

test=release/1.5

Fix #17955
Fix bug in sequence_unpad op, when allocate the output memory do not match actual memory, check memory failed. Fix this bug by allocating the output memeory in correct code position.

fix merge conflict of 'Remove attribute in Allocator::Allocate' and elementwise_add_mkldnn_op (#17949)

test=develop

* remove attribute in Allocator::Allocate, test=develop

* fix travis ci error, test=develop

* Enable seq_pool op to accept len 0 input

test=develop

* Update sequence_pool's api

test=develop

* Add more unittest cases for seq_pool op

test=develop

* Remove legacy comments

test=develop

* Don't use template in op maker

test=develop

test=develop

* add reshape support tensor; test=develop

* fix reshape bug; test=develop

* change reshape attribute default value; test=develop

* fix reshape input name; test=develop

* fix reshape unitest; test=develop

* check dim tensor shape; test=develop

test=develop

test=develop

fix bug: the lod_tensor_to_array op will aplly a new var but not release when dong inference (#17856)

test=develop

* Enable ngraph layer_norm operator

test=develop

* Disable/Enable cuda, new unit-test test=develop

* Fix use_cudnn test=develop

* Fixed test_layer test, new funciton is added test=develop

* set use_cudnn by default test=develop

* delay infershape test=develop

* fall back subblock to paddle test=develop

* fix edge cases test=develop

* remove output duplicates test=develop

* handle reshape2_grad infershape test=develop

* expand support tensor attribute; test=develop

* fix bug ; test=develop

* fix uni test bug; test=develop

* fix copy bug; test=develop

* refine expand_times default value; test=develop

* support some input tensor remain on cpu; test=develop

* fix input = none; test=develop

* fix unfound bug; test=develop

* fix proto None case; test=develop

* fix bug; test=develop

* fix proto null bug; test=develop

* remove conv check; test=develop

* fix test bug; test=develop

* move fill constant; test=develop

* no change in proto; test=develop

* fix bug; test=develop

* change attr detph name; test=develop

* remove remain cpu; test=develop

* fix bug; test=develop

* merge develop; test=develop

* fix one_hot bug; test=develop

* fix bug; test=develop

* fix bug; test=develop

* fix bug; test=develop

* fix python api bug; test=develop

Refactor the code to be OpenMP friendly

test=develop

* revise conv layer cudnn algo choose test=develop

* update for code style test=develop

* update for code style test=develop

* Enable elementwise_div operator test=develop

* Fix update date test=develop

* fix the bug of mobilenet-ssd INT8 inference without overloading GetHash
test=develop

* remove the out_grad->format() in TransposeMKLDNNGradOpKernel
test=develop