`batchsize=Inf` or something?

It would be nice if you could `DataLoader` for one maximal-size batch, without knowing the size of the inputs.

This would mean that a function which loads some data, pre-processes it, and then returns a `DataLoader` could easily be used to return the full dataset, in the identical format, as long as it passes the keyword `batchsize` along.

Could be `batchsize=0`, since `-1` already does something special. Although unfortunately 0 is not an error right now.