在tensorflow中`_softmax_cross_entropy_with_logits`是在哪里定义的？

def _softmax_cross_entropy_with_logits(logits, labels, name=None):
    # 计算softmax交叉熵损失
    with ops.name_scope(name, "softmax_cross_entropy_loss", [logits, labels]):
        logits = ops.convert_to_tensor(logits, name="logits")
        labels = math_ops.cast(labels, logits.dtype, name="labels")
        labels = _remove_squeezable_dimensions(labels, logits)
        logits.get_shape().assert_is_compatible_with(labels.get_shape())

        # 计算softmax
        softmax = nn_ops.softmax(logits, name="softmax")

        # 计算交叉熵损失
        loss = nn_ops.softmax_cross_entropy_with_logits(labels=labels, logits=logits, name="cross_entropy_loss")

        return array_ops.reshape(loss, array_ops.shape(labels), name="loss")