* Split some APIs from nn.py to loss.py

test=develop

* fix test_detection unit-test

test=develop

* refine 13 en doc

* Polish En doc for some APIs
* Update some comments and API.spec

Fix english doc api: sequence_slice, sigmoid_cross_entropy_with_logits, ssd_loss, DetectionMAP, detection_map (#20405)

* Fix english doc api: sequence_slice, sigmoid_cross_entropy_with_logits, ssd_loss, DetectionMAP, detection_map

* modify document of 11 APIs
test=develop
test=document_fix

* fix dtype to data type and description of name parameter

Fix english doc api, invloves the op of retinanet_target_assign, sigmoid_focal_loss and retinanet_detection_output. (#20250)

* test=document_fix
test=develop
Fix english doc api, invloves the op of retinanet_target_assign, sigmoid_focal_loss and retinanet_detection_output.

* test=document_fix
test=develop
remove Notice: this OP supports CPU mode only in english doc api of retinanet_target_assign and retinanet_detection_output

* test=document_fix
test=develop
fix API Difference for retinanet_target_assign and retinanet_detection_output

* fix API.spec conflicts in english doc api of retinanet_target_assign, sigmoid_focal_loss,  retinanet_detection_output
test=develop
test=document_fix

* fix API.spec conflicts in english doc api of retinanet_target_assign, sigmoid_focal_loss, retinanet_detection_output
test=develop
test=document_fix

* polish en api,test=develop,test=document_fix

* modified api.spec, test=develop,test=document_fix

* modified ops for arc,test=develop,test=document_fix

* update for 1.6, test=develop,test=document_fix

* update API.spec

* Update API.spec

test=develop,test=document_fix

* Update API.spec

test=develop,test=document_fix

* refine Normal Uniform documnet

* fix fluid.data code example, test=develop, test=document_preview, test=document_fix

* use None instead of -1 in shape, test=develop, test=document_preview, test=document_fix

* refine doc, test=document_fix

* add API.spec,test=develop,test=document_fix

* update API.spec

* merge_empty_lod_tensor, test=develop

* fix multiclass_nms, test=develop

* refine API.spec, test=develop

* add unittest case for fetch, test=develop

* add lod tensor test, test=develop

* return index for multiclass_nms, test=develop

* add api for multiclass_nms2

* update API.spc, test=develop

* refine api doc, test=develop

* fix test_detection.py, test=develop

* polish code, test=develop

* add more unittest case, test=develop

test=develop, test=document_preview

test=develop, test=document_preview

* fix transform matrix bug, test=develop

* modify API.spec

* modify roi_perspective_transform_op to output mask and transform matrix

* modify comment

* modify comment

* modify API.spec

* update API.spec

* remove no use header, test=develop

* resolve conflict

* Simplify multi_box_head API in detection.py and remove assign op.

* fix spelling errors; test=develop

* Update API.spec

update md5

* Update API.spec

* change the order of api;test=develop

* test=develop
add detection output for supporting retinanet

* test=develop
add test_layers.py

* test=develop
add API.spec

* test=develop
alter test_retinanet_detection_output.py

* test=develop
alter round 2

* test=develop
alter retinanet_detection_output

* test=develop
alter paddle/fluid/API.spec

* test=devlop
alter detection.py

* test=develop
alter retinanet_detection_output

* test=develop
alter paddle/fluid/API.spec

* test=develop
alter detection.py

* test=develop
alter API.spec

* test=develop
alter retinanet_detection_output

* test=develop
alter paddle/fluid/API.spec

* test=develop
alter python/paddle/fluid/tests/unittests/test_retinanet_detection_output.py

* test=develop
alter python/paddle/fluid/tests/unittests/test_retinanet_detection_output.py

* test=develop
fix grammer error

* test=develop
fix grammer error

* test=develop
fix grammer error

* test=develop
alter python/paddle/fluid/tests/unittests/test_layers.py

* test=develop
alter paddle/fluid/API.spec

* test=develop
add sigmoid_focal_loss for supporting retinanet

* test=develop
add test_layers

* test=develop
add API.spc

* test=develop
alter sigmoid_focal_loss_op.cc

* test=develop
alter detection.py

* test=develop
alter API.spec

* test=develop
alter round 1

* test=develop
alter simooid_focal_loss

* test=develop
alter sigmoid_focal_loss_op.cc

* test=develop
alter test_layers.py

* test=develop
alter paddle/fluid/API.spec

* test=develop
alter sigmoid_focal_loss_op.cu

* test=develop
alter paddle/fluid/operators/detection/sigmoid_focal_loss_op.cc

* Update generate_proposal_labels_op to support CascadeRCNN.

* test=develop add target assign for retinanet

* test=develop
run ci

* test=developp
add test_layers

* test=develop
add APi.spec

* test=develop
alter round 1

* test=develop
alter rpn_target_assign_op.cc

* test=develop
alter test_rpn_target_assign_op.py

* test=develop
alter rpn_target_assign_op.cc

* test=develop

alter API.spec

* test=develop
alter paddle/fluid/operators/detection/rpn_target_assign_op.cc

* test=develop
alter rpn_target_assign_op.cc

* test=develop
alter python/paddle/fluid/layers/detection.py

* test=develop
alter paddle/fluid/API.spec

* Remove layers.detection_map API
* Since uers can use fluid.metrics.DetectionMAP to calculate mAP of current-batch and cumulative-batch. layers.detection_map only can calculate cur-batch mAP.

* add collect fpn proposals op,test=develop

* Fix the example code in some Python API
* Update paddle/fluid/API.spec
* Fix some examples format

* refine api comment, test=develop

* fix unexecutable API comments, test=develop

* add API.spec,test=develop

* fix distribute fpn proposals, test=develop