Recursively put `b` on `device`.

def to_device(b:Tensors, device:torch.device):
    "Recursively put `b` on `device`."
    device = ifnone(device, defaults.device)
    if is_listy(b): return [to_device(o, device) for o in b]
    if is_dict(b): return {k: to_device(v, device) for k, v in b.items()}
    return b.to(device, non_blocking=True)