[BC-breaking] Update KeypointRCNN weights

[fmassa]

Fixes #1606

We support old weights by passing pretrained='legacy', but the default behavior is to pick the new set of weights.
As such, this is a BC-breaking change.

So if users relied on the pretrained features from KeypointRCNN for training classifiers (and only saved the classifiers, picking the weight from torchvision pretrained model), they will need to change to pretrained='legacy'.

[codecov-io]

Codecov Report

Merging #1609 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1609      +/-   ##
==========================================
- Coverage   65.65%   65.63%   -0.03%     
==========================================
  Files          90       90              
  Lines        7085     7088       +3     
  Branches     1075     1076       +1     
==========================================
  Hits         4652     4652              
- Misses       2126     2129       +3     
  Partials      307      307

Impacted Files	Coverage Δ
torchvision/models/detection/keypoint_rcnn.py	78.26% <0%> (-3.56%)	⬇️

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