Merge branch 'develop'.

lstmp error message enhancement.
sequence_conv error message enhencement.
sequence_enumerate error message enhencement.
sequence_mask error message enhencement.

This reverts commit 29f1560d.

* add xpu in heter mode
test=develop

* BOOST_CONST_GET; PADDLE_THROW
test=develop

* code style
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* code style
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* code style
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* refine
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* refine
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* refine
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* refine code
test=develop

* fix bn double grad, test=develop

* update, test=develop

* try to fix paddle save load test

* open paddle save load

* replace dataloader

* remove dataloader

* add fp16 for matmul

add abs support double grad for the api 2.0

* refine PADDLE_ENFORCE

* [buf fix]:fix unittest test_activation_op error

* split long-time unittests to smaller ones

* rename some unittests

* fix unittest_group_norm_op_v2, test=develop

* fix unittest_group_norm_op_v2, test=develop

fix cuda atomic for ARCH<350 for the automic_max

* Add pass compatibility check for four mkldnn pass, test=develop

* modify ci api white list, add annotation

* modify ci api white list, add annotation

* move gpu_not_white list to wlist.json

* add null line

* update gcc8 in python3 ci docker

* change cuda 10.2

* update cudnn8

* nvidia error cuda10.2-cudnn8-centos6 images

* fix third cache

* add unittest count of windows

* Reduce the number of retries

* make places of DataLoader optional. test=develop

* test=develop, bug fix

As the title, decrease random failure probability for test_parallel_executor_mnist

The old code set larger delta when comparing reduce and all reduce, but didn't set all. I added it.

On my linux machine, I run 100 times, no failure occurs. In addition, we only saw this random failure on CI two times since I worked. I thought it was rare and I just increased the delta.

* fix error message in conv/conv_transpose. test=develop

* refine err msg in shuffle channel op

* fix approveals message;test=document_fix

* fix approveals message;test=document_fix

* fix tensorrt 6 build error. test=develop

* fix. test=develop

* bug fix

* test=develop