[Feature] Release new fp16-dcn models ( mask-rcnn DCN/DCNv2)

[AronLin]

Motivation

Release mask_rcnn_r50_fpn_fp16_dconv_c3-c5_1x_coco and mask_rcnn_r50_fpn_fp16_mdconv_c3-c5_1x_coco,
including config files, models and log files.

it is based on open-mmlab/mmcv#1029.

Modification

Add two config files.
Release models.
Modify the readme file.

BC-breaking (Optional)

If pytorch >= 1.6.0 with that the mmcv version is larger than 1.3.2 and before open-mmlab/mmcv#1029 merged, there will be a type mismatch bug in fp16 training.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #5201 (35febe6) into master (3d91b8b) will decrease coverage by 0.86%.
The diff coverage is n/a.

❗ Current head 35febe6 differs from pull request most recent head 818e9a8. Consider uploading reports for the commit 818e9a8 to get more accurate results

@@            Coverage Diff             @@
##           master    #5201      +/-   ##
==========================================
- Coverage   65.18%   64.32%   -0.87%     
==========================================
  Files         272      272              
  Lines       21078    20933     -145     
  Branches     3531     3531              
==========================================
- Hits        13740    13465     -275     
- Misses       6570     6687     +117     
- Partials      768      781      +13     

Flag	Coverage Δ
unittests	64.31% <ø> (-0.84%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet/models/necks/yolo_neck.py	62.00% <0.00%> (-38.00%)	⬇️
mmdet/models/dense_heads/yolo_head.py	27.90% <0.00%> (-23.25%)	⬇️
mmdet/core/export/onnx_helper.py	13.18% <0.00%> (-19.08%)	⬇️
mmdet/models/dense_heads/fcos_head.py	67.84% <0.00%> (-7.05%)	⬇️
mmdet/models/dense_heads/rpn_test_mixin.py	77.41% <0.00%> (-6.46%)	⬇️
mmdet/core/bbox/coder/delta_xywh_bbox_coder.py	94.89% <0.00%> (-5.11%)	⬇️
mmdet/models/utils/normed_predictor.py	88.09% <0.00%> (-4.77%)	⬇️
mmdet/core/bbox/transforms.py	75.89% <0.00%> (-4.47%)	⬇️
mmdet/models/dense_heads/anchor_head.py	83.62% <0.00%> (-3.10%)	⬇️
mmdet/models/detectors/base.py	52.54% <0.00%> (-2.66%)	⬇️
... and 54 more

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