Body model feature map number 19.

Hi, I noticed the body model return a tensor with 19 channels.
Adding ```print("Mconv7_stage6_L2", Mconv7_stage6_L2.shape)``` in ```openpose/body/estimator.py``` on line 139 will print ```Mconv7_stage6_L2 torch.Size([1, 19, 23, 17])``` for the demo image.
The openpose model has only 18 keypoints.
What is channel 19 for ? 

Best,
Pierre