Enhance torchvision's draw_keypoint method

🚀 The feature

Follow up of #4216 . The current utility works fine, but we can make it better :)

Motivation, pitch

I think the following can be added.

• Support multiple colors

There were 2 proposals one by @fmassa and one by me. The problem was handling excess colors.
I think that in fmassa's version we can broadcast to bigger dimensions and make it work?

• Support specifying line color.

In my opinion, we shouldn't complicate this and just allow a single string.
Happy to hear thoughts though.

• Plot the circle above the line or vice versa?

We currently plot the line above the circle. So at places I see that the line comes above the circle, somehow it looks like the line cuts / bisects the circle. I feel we should do the opposite.

• Don't use default white for all colors. Use either a random palette / better default one.

I guess we should extend this functionality to all plotting utils, by default both draw_bounding_boxes and draw_keypoints plot with white color. While segmentation masks uses a fixed color palette.
Either we could use random colors / just have a nice fixed default palette.
(Also see #4658 )

cc @fmassa @datumbox @NicolasHug

Start by locating torchvision's draw_keypoint method and the related draw_bounding_boxes and segmentation-mask plotting utilities. Read the discussion in #4216 and #4658, including the proposals about color handling. Done means an agreed subset of the listed enhancements is implemented with its behavior clearly defined.