Split mask_rcnn.py into several files

[fmassa]

mask_rcnn.py was too big, and contained implementations for both Faster R-CNN and Keypoint R-CNN that didn't need to be there.

[codecov-io]

Codecov Report

Merging #921 into master will increase coverage by 0.17%.
The diff coverage is 85.62%.

@@            Coverage Diff             @@
##           master     #921      +/-   ##
==========================================
+ Coverage   60.59%   60.77%   +0.17%     
==========================================
  Files          60       63       +3     
  Lines        4926     4948      +22     
  Branches      733      733              
==========================================
+ Hits         2985     3007      +22     
  Misses       1752     1752              
  Partials      189      189

Impacted Files	Coverage Δ
torchvision/models/detection/backbone_utils.py	100% <100%> (ø)
torchvision/models/detection/mask_rcnn.py	87.03% <100%> (+3.09%)	⬆️
torchvision/models/detection/__init__.py	100% <100%> (ø)	⬆️
torchvision/models/detection/faster_rcnn.py	77.92% <77.92%> (ø)
torchvision/models/detection/keypoint_rcnn.py	88.33% <88.33%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2e1e0b6...64de986. Read the comment docs.