Support 'shifting' when training a model

[Ezward]

As many people already mentioned, training on the previous image, or the next angle/throttle, i.e. shifting the y in (x,y) forward by one index, gives a better and forward looking behavior which looks like having more "planning" and less "observe and react if you can". Given that the inference time makes up the largest part of a single vehicle loop, it does make sense to use the shift. It is easy to implement, because we support the sequence models which grab a continuous list of (x, y), so we can adjust this to grab a set of two consecutive data and use image from the first and steering, throttle from the second entry.

Shifting my a factor of 4 helps for online races anecdotally (compensating for frame lag)
Shifting in general improves fit quite substantially so it's definitely worth adding and you probably want it configurable