最大熵模型中的heldout accuracy（張樂老師提供的最大熵模型包相關技術）
裏面有一段這樣的描述，我想了解一下有關heldout的相關詳情，並且爲什麼會因此引入高斯方差？謝謝！
In this example, it seems performance peaks at iteration 9. Further training actually brings down the accuracy on
the heldout data, although the training accuracy continues to increase. Applying a Gaussian prior can help avoid
overfitting, just use -g float to specify the global Gaussian variance σ 2 .
