Add crestereo dataset

[TeodorPoncu]

Added a Stereo Matching Dataset interface similar to the Optical Flow one. I believe we will need to have a renaming of datasets based on the task, at least for 2.5D related, as we might have naming clashes if we plan on adding Depth Estimation as well. Since datasets such as Kitti or FallingThings or SceneFlow can be used for multiple of these tasks.

I've also set the outputs of the disparity map and valid mask to (1, H, W), (H, W) to be aligned with the way the Flow datasets output a flow in shape (2, H, W) and a mask in (H, W) where possible.

[YosuaMichael]

I close this PR since it is replaced by few other smaller PRs already