This PR fixes a memory issue when running video analysis using a model with an identity head, as reported in #2771.

Improvements

• PadOutputs: instead of needing to set "key": 0 for outputs we don't want to pad (such as identity_heatmap), check if each output is present in the max_individuals dict to see if padding is needed.
• PredictKeypointIdentities: Models with identity heads would output the identity heatmaps, which leads to memory issues when analyzing large videos. A keep_id_maps=False parameter is added, only keep identity heatmaps in the output when explicitly requested.
• assign_identity: use the pose confidence to weight identity scores when assigning identity, as done in the TensorFlow implementation

Bug fixes

• make_pytorch_test_config when there are unique bodyparts for a project. The unique bodyparts need to be included in all_joint_names and all_joints, otherwise video analysis fails.