Compute accuracy when `y_pred` and `y_true` are the same size.

def accuracy_thresh(y_pred:Tensor, y_true:Tensor, thresh:float=0.5, sigmoid:bool=True)->Rank0Tensor:
    "Compute accuracy when `y_pred` and `y_true` are the same size."
    if sigmoid: y_pred = y_pred.sigmoid()
    return ((y_pred>thresh)==y_true.byte()).float().mean()