[cherry-pick]polish doc for uniform_random and multi_box_head

[Cherry-pick] polish yolov3_loss annotation

add continuous value model op

refine roi align infer shape. test=release/1.4

[cherry-pick] Fix load persistables in graph wrapper.

fix split_byref_op infer shape

Merge pull request #16746 from xjqbest/dataset_merge_develop

Merge pull request #16652 from xjqbest/dataset_merge_develop

test=develop

fix save and load bug, test=release/1.4

test=release/1.4

Enable setting thread number in Anakin Config

Fix code style

Reduce cmake file changes

test=release/1.4

imperative fix train mode

test=develop

[cherry-pick] fix yolo param

move split filelist from trainer.py to fleet & fix error

fix dataset bug

test=release/1.4

test=release/1.4

Fix anakin subgraph shufflenet

test=develop

test=release/1.4 cherry-pick (#16845)

Imperative untrack op in eval mode

fix model_stat import bug

test=develop

* optimize lstmp and sample_logits op, test=develop

* update op_use_default_grad_op_maker.spec, test=develop

* delete useless file,test=develop

* append 0 to dim variable to avoid memory reusage, test=develop

* test=release/1.4, fix hsigmoid dereference nullptr

* test=release/1.4, refine condition

* test=release/1.4, refine comments

test=release/1.4

cherry-pick dygraph serenext unit test

test=release/1.4

fix the hang bugs of memory copying. test=develop