* sequential reader stage 1, test=develop

* fix ut, test=develop

* fix iterable=False reset bug, add some logs and polish code, test=develop

* inference feed partial data, test=develop

* Turn on keep_order=True for test, test=develop

* enhance ut to test more cases, test=develop

* test commit for reverting

* Revert "test commit for reverting", test=develop

This reverts commit 80aef42e.

* add ut of merged and unmerged results, test=develop

* add more uts for coverages and add en doc of api, test=develop

* follow comments, test=develop

* change note style, test=develop

update embedding_eltwise_layernorm fuse pass and fused kernel, to support multi input

* 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

* remove unnecessary prepare data, test=develop

* Op in while block will not skip PrepareData, test=develop

* add fusion group test for backward and refine code

* add support for expression type convert and add cast Op support in fusion group

* add skip_layernorm pass. test=develop

* 1. add embedding eltwise layernorm fuse
2. add embedding eltwise layernorm op
3. refine inplace_add_relu
4. refine fc_eltwise_layernorm
test=develop

* 1. refine fc
test=develop

* fix comments
test=develop

* fix comments

test=develop

* refine grad maker, test=develop

* refactor tracer stage 1, test=develop

* merge develop to solve conflict third times, test=develop

* fix fc padding during fusion, test=develop

* fix optim model inference after SaveOptimModel, test=develop

* refine the profiler print test=develop

* reduce default attrs for dynamic graph, test=develop

* add some explanations for explicit attr, test=develop

* tweak explicit attr comments, test=develop

* add fluid.device_guard to specify the device type for Op

* update ScopeBufferedSSAGraphExecutor&AsyncSSAGraphExecutor&ThreadedSSAGraphExecutor&FastThreadedSSAGraphExecutor&ParallelSSAGraphExecutor&ParallelExecutor for fetching unmerged results.

* add the unit test for fetch_unmerged.

* update ut for multi-card and multi-cpu.

* add the error message and the user suggestion in FetchOpHandle. test=develop

* user can call dataset.set_download_cmd to set its customized download cmd
* add UT to cover this scenario

* Add the codegen and auto fusion for sum Op  in fusion group

* support cond in clone, test=develop

* refine code, test=develop

* refine code, test=develop

* follow comments, test=develop

* refine code, test=develop

* Add two types of Metric Calculator: MultiTaskCalculator & CmatchRankCalculator.
* Add a config for DynamicAdjustChannelNum function to denote whether we will discard the remaining instances when they are not be distributed evenly.
* Remove CPU code in Pull/PushSparse and we will add it back when testing it fully.
* Fix some known issues: such as copying persistable vars after one epoch running.

* Add an interface of disabling FC padding
* fix bert regression
* polish fc padding interface
* recover pass function
* fix argument error
* fix mkldnn error

* add sync communicator and implement

* add python flag to control profile level test=develop

当一个模型中有多个fc_lstm子图的时候，且其中fc共用了同一个persistable的bias，此时不应该将bias节点删除，只将非persistable的节点去除即可。

* 1. optim multihead matmul: fuse three fc to multihtead matmul

test=develop

* fix conflict
test=develop

* fix comments
test=develop

test=develop

* add thread barrier for the compiled program

Refine PaddleBox Framework, Main functions: 
* Add MetricMsg util class, which can calculate metrics like AUC, bucket_error, COPC.
* Replace FeedPass with new interface: BeginFeedPass & EndFeedPass
* Refactor Pull/Push Sparse Function in box_wrapper.
* Use CUDA Kernel to copy keys and copy feasign between tensor and boxps struct.
* Cache copied keys in pull sparse in order to reuse it in push period.