Applies TTA to predict on `ds_type` dataset.

def _TTA(learn:Learner, beta:float=0.4, scale:float=1.35, ds_type:DatasetType=DatasetType.Valid, with_loss:bool=False) -> Tensors:
    "Applies TTA to predict on `ds_type` dataset."
    preds,y = learn.get_preds(ds_type)
    all_preds = list(learn.tta_only(scale=scale, ds_type=ds_type))
    avg_preds = torch.stack(all_preds).mean(0)
    if beta is None: return preds,avg_preds,y
    else:            
        final_preds = preds*beta + avg_preds*(1-beta)
        if with_loss: 
            with NoneReduceOnCPU(learn.loss_func) as lf: loss = lf(final_preds, y)
            return final_preds, y, loss
        return final_preds, y