[Feature] add a score_per_joint option in test_cfg

[piercus]

Related to #9 (comment)

Describe the feature

It would be great to have a 'score_per_joint' option in test_cfg in order to output one score per joint, instead of having only a global score for the pose, my use case is related to associative embedding

Motivation

In the context of bottom-up models, currently, we have 2 options :

• return_heatmap=False will only return the pose with one score per pose
• return_heatmap=True will give the full details about heatmaps as output

1. Those 2 choices are inconvenient if you do not want to carry the whole heatmaps, and you still want to know exaclty which joint, in the pose is present for sure, and which joint might be occluded, it is even more the case when using refine=True and missing joint are completed, loosing the information about which joint is confident and which one is not.

2. I feel that all the use cases related to detecting occluded parts vs non-occlueded parts may benefit from this feature