Remove/move 16 fluid APIs (PaddlePaddle#48377)

* remove density_prior_box

* remove anchor_generator

* remove roi_perspective_transform

* remove generate_proposal_labels

* remove generate_mask_labels

* remove generate_proposals

* remove box_clip

* remove retinanet_detection_output

* remove multiclass_nms

* remove locality_aware_nms

* remove matrix_nms

* remove distribute_fpn_proposals

* remove box_decoder_and_assign

* remove collect_fpn_proposals

* remove 2 trt files

* move prior_box to static/nn/common.py

* move multi_box_head to static/nn/common.py

* fix for CI/CE

* remove retinanet_detection_output

* restore compile_vs_runtime_white_list.py

* restore test_retinanet_detection_output to white list

* replace nn.flatten by paddle.flatten, and fix doc for retinanet_target_assign

* add enable_static in demo and fix bug

* remove roi_perspective_transform in test_layers

* remove multi_box_head

* change self.multiclass_nms to _legacy_C_ops.multiclass_nms

* empty commit

* empty commit

* check code style

* fix prior_box

* fix CI

* remove redundant prior_box in detection.py

* fix docs

* remove detection

* fix prior_box en doc

* delete prior_box in common

* remote proir_box from __init__.py

python/paddle/fluid/evaluator.py

@@ -21,7 +21,6 @@
 from . import unique_name
 from .layer_helper import LayerHelper
 from .initializer import Constant
-from .layers import detection
 
 
 def _clone_var_(block, var):

python/paddle/fluid/layers/__init__.py

@@ -24,8 +24,6 @@
 from .math_op_patch import *
 from . import loss
 from .loss import *
-from . import detection
-from .detection import *
 from .learning_rate_scheduler import *
 from .collective import *
 from .sequence_lod import *
@@ -36,7 +34,6 @@
 __all__ += io.__all__
 __all__ += tensor.__all__
 __all__ += control_flow.__all__
-__all__ += detection.__all__
 __all__ += learning_rate_scheduler.__all__
 __all__ += sequence_lod.__all__
 __all__ += loss.__all__