update RTMPose `SimCCPredictor`: expose `apply_softmax` and fix visibility thresholding (#3306)

* fix visibility thresholding for `apply_softmax=True`

the original visibility thresholding `locs[vals <= 0] = 1` makes sense for unnormalized logits, but after softmaxing this becomes effectively a no-op. The current commit changes this to a reasonable threshold for the case when apply_softmax=True

* rtmpose: expose default parameters in config yaml (e.g. `apply_softmax`)

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Co-authored-by: Cyril Achard <cyril.achard@epfl.ch>
Co-authored-by: Mackenzie Mathis <mathis@rowland.harvard.edu>