【转载】 深度学习总结：用pytorch做dropout和Batch Normalization时需要注意的地方，用tensorflow做dropout和BN时需要注意的地方,

原文地址：

https://blog.csdn.net/weixin_40759186/article/details/87547795

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用pytorch做dropout和BN时需要注意的地方

pytorch做dropout:

就是train的时候使用dropout,训练的时候不使用dropout,
pytorch里面是通过net.eval()固定整个网络参数，包括不会更新一些前向的参数，没有dropout，BN参数固定，理论上对所有的validation set都要使用net.eval()
net.train()表示会纳入梯度的计算。

net_dropped = torch.nn.Sequential(
    torch.nn.Linear(1, N_HIDDEN),
    torch.nn.Dropout(0.5),  # drop 50% of the neuron
    torch.nn.ReLU(),
    torch.nn.Linear(N_HIDDEN, N_HIDDEN),
    torch.nn.Dropout(0.5),  # drop 50% of the neuron
    torch.nn.ReLU(),
    torch.nn.Linear(N_HIDDEN, 1),
)

for t in range(500):
    pred_drop = net_dropped(x)
    loss_drop = loss_func(pred_drop, y)

    optimizer_drop.zero_grad()
    loss_drop.backward()
    optimizer_drop.step()

    if t % 10 == 0:
        # change to eval mode in order to fix drop out effect
        net_dropped.eval()  # parameters for dropout differ from train mode

        test_pred_drop = net_dropped(test_x)

        # change back to train mode
        net_dropped.train()

pytorch做Batch Normalization:

net.eval()固定整个网络参数，固定BN的参数，moving_mean 和moving_var，不懂这个看下图:

            if self.do_bn:
                bn = nn.BatchNorm1d(10, momentum=0.5)
                setattr(self, 'bn%i' % i, bn)   # IMPORTANT set layer to the Module
                self.bns.append(bn)

    for epoch in range(EPOCH):
        print('Epoch: ', epoch)
        for net, l in zip(nets, losses):
            net.eval()              # set eval mode to fix moving_mean and moving_var
            pred, layer_input, pre_act = net(test_x)

            net.train()             # free moving_mean and moving_var
        plot_histogram(*layer_inputs, *pre_acts)  

moving_mean   和   moving_var

用tensorflow做dropout和BN时需要注意的地方

dropout和BN都有一个training的参数表明到底是train还是test, 表明test那dropout就是不dropout，BN就是固定住了BN的参数；

tf_is_training = tf.placeholder(tf.bool, None)  # to control dropout when training and testing

# dropout net
d1 = tf.layers.dense(tf_x, N_HIDDEN, tf.nn.relu)
d1 = tf.layers.dropout(d1, rate=0.5, training=tf_is_training)   # drop out 50% of inputs

d2 = tf.layers.dense(d1, N_HIDDEN, tf.nn.relu)
d2 = tf.layers.dropout(d2, rate=0.5, training=tf_is_training)   # drop out 50% of inputs

d_out = tf.layers.dense(d2, 1)

for t in range(500):
    sess.run([o_train, d_train], {tf_x: x, tf_y: y, tf_is_training: True})  # train, set is_training=True

    if t % 10 == 0:
        # plotting
        plt.cla()
        o_loss_, d_loss_, o_out_, d_out_ = sess.run(
            [o_loss, d_loss, o_out, d_out], {tf_x: test_x, tf_y: test_y, tf_is_training: False} # test, set is_training=False
        )

    def add_layer(self, x, out_size, ac=None):
        x = tf.layers.dense(x, out_size, kernel_initializer=self.w_init, bias_initializer=B_INIT)
        self.pre_activation.append(x)
        # the momentum plays important rule. the default 0.99 is too high in this case!
        if self.is_bn: x = tf.layers.batch_normalization(x, momentum=0.4, training=tf_is_train)    # when have BN
        out = x if ac is None else ac(x)
        return out

当BN的training的参数为train时，只是表示BN的参数是可变化的，并不是代表BN会自己更新moving_mean 和moving_var，因为这个操作是前向更新的op,在做train之前必须确保moving_mean 和moving_var更新了，更新moving_mean 和moving_var的操作在tf.GraphKeys.UPDATE_OPS

        # !! IMPORTANT !! the moving_mean and moving_variance need to be updated,
        # pass the update_ops with control_dependencies to the train_op
        update_ops = tf.get_collection(tf.GraphKeys.UPDATE_OPS)
        with tf.control_dependencies(update_ops):
            self.train = tf.train.AdamOptimizer(LR).minimize(self.loss)