Visualize feature maps from the contrastive encoder

[ziw-liu]

Added a script to perform PCA along the channel dimension to visualize multi-scale feature maps in the contrastive model's encoder stages.

[ziw-liu]

Not urgent to merge at this moment.

[mattersoflight]

@ziw-liu, feature visualization vs. time didn't serve the purpose in early iterations. Did you also try it after we started training time-aware models? The temporal smoothness in the output layer does not enforce the temporal smoothness in intermediate layers. This is interesting.