Change Clay forward signature to work with tasks

[fmartiescofet]

All tasks assume the batch returned by the dataloader is a dictionary with an "image" key, the corresponding y label (either "mask" or "label") and the rest of elements of the dictionary are passed as keyword arguments to the model, see an example here.
For this reason, it was not possible to pass the metadata to Clay using a dictionary (which is what Clay expected).

This PR modifies Clay to accept the metadata as keyword arguments instead of a dict and the Embedder constructs the dictionary using a Datacuber.

It also fixes a computation of the positional embedding when a device different than cpu is used.