Make vec<Tensor> can be serialized to RecordIO

[Speed]Refine elementwise_mul_op

* test cpu float16 data transform

* add isnan etc

* small fix

* fix containsNAN test error

* add data_type transform GPU test

* add float16 GPU example

* fix error

* fix GPU test error

* add context wait

* Redesign channel implementation for Select Op

* Remove unecessary header

* Remove unnecessary comments

fix snappy build

* Fix mAP evaluator bug.

* Fix bug in detection mAP evaluator.

* Fix unit testing.

* Support to set background label index in detection mAP op.

Feature/recordio

Add ceil_mode option for pool2d and pool3d

Fix nullptr when doing nested profiling

fix document deployment

[Speed]Avoid init_nccl for every steps.

Add program cache for executor.py

update cluster_train page

* fix is_test issue

* add paddle enforce

* fix bug

* add new func

* small fix

* address comments

Merge branch 'develop' of https://github.com/PaddlePaddle/Paddle into add-program-cache-for-executor

* change learning_rate_decay to learning_rate_scheduler
* optimize code
* change nn.cast to tensor.cast

Create new structure for V2 and fluid doc

Implement detection mAP evaluator wrapper and unify label format between SSD loss and mAP evaluator  (#8736)

* Implement mAP evalutor Python interface.

* Fix unit testing and uniy label format between SSD loss and mAP evalutor.

* Update doc.

* add write_new_layers in rst tree