论文：TPAMI 2023 图神经网络的强化因果解释器

论文代码地址：代码

目录

Abstract

Introduction

PRELIMINARIES

Causal Attribution of a Holistic Subgraph​

individual causal effect (ICE)​

*Causal Screening of an Edge Sequence

Reinforced Causal Explainer (RC-Explainer)​

Policy Network

Policy Gradient Training

Discussion

EXPERIMENTS

Evaluation Metrics

Evaluation of Explanations​

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Abstract

Motivation:解释图神经网络(GNNs)的预测结果来理解模型决策背后的原因。现有Feature attribution忽略了边之间的依赖关系，尤其是协同效应。

Method：引入Reinforced Causal Explainer(RC-Explainer)实现因果筛选策略, 策略网络学习边序列生成策略(每个边缘被选中的概率)，在每step选择一个潜在边缘作为action,获得由每个边的组合和子图的因果属性组成的reward，可突出解释边的依赖性、边的联盟的影响。

用策略梯度来优化策略网络，并通过对GNN全局理解，RC-Explainer能为每个图实例提供模型级解释，并泛化到未见过的图。

Conclusion:在解释三个图分类数据集上不同的GNN时，RC-Explainer在predictive accuracy、contrastivity等两个定量指标上实现了与最先进方法相当或更好的性能，并通过了合理性检查(sanity checks)和视觉检查(visual inspections)。

 一、Introduction

PRELIMINARIES

相关代码实现：Mutag_gnn.py

节点表示：

#获取节点表示
    def get_node_reps(self, x, edge_index, edge_attr, batch):
        node_x = self.node_emb(x)#节点嵌入层
        edge_attr = self.edge_emb(edge_attr)#边嵌入层
        # 对于每个 GINConv 单元
        for conv, batch_norm, ReLU in \
                zip(self.convs, self.batch_norms, self.relus):
            node_x = conv(node_x, edge_index, edge_attr)              #节点表示传递给GINConv层进行信息聚合
            node_x = ReLU(batch_norm(node_x))#标准化，激活函数
        return node_x

最终用于预测的表示： 

def get_graph_rep(self, x, edge_index, edge_attr, batch):
        node_x = self.get_node_reps(x, edge_index, edge_attr, batch)
        graph_x = global_mean_pool(node_x, batch)
        return graph_x

def get_pred(self, graph_x):
        pred = self.relu(self.lin1(graph_x))#线性层，relu处理图表示
        pred = self.lin2(pred)#预测
        self.readout = self.softmax(pred)
        return pred

Causal Attribution of a Holistic Subgraph

individual causal effect (ICE)

论文代码中对于互信息的实现，在reward的计算中

def get_reward(full_subgraph_pred, new_subgraph_pred, target_y, pre_reward, mode='mutual_info'):
    if mode in ['mutual_info']:
        #计算互信息，衡量完整子图预测值和新子图预测值之间的相似度
        # full_subgraph_pred:[batch_size, num_classes] reward:[batch_size]
        reward = torch.sum(full_subgraph_pred * torch.log(new_subgraph_pred + EPS), dim=1)
        #对每个样本，新子图预测的最大类别与目标类别相同+1；否则-1
        reward += 2 * (target_y == new_subgraph_pred.argmax(dim=1)).float() - 1.
        # print('reward2',reward)
    elif mode in ['binary']:
        # 新子图预测的最大类别与目标类别相同，奖励+1；否则-1
        reward = (target_y == new_subgraph_pred.argmax(dim=1)).float()
        reward = 2. * reward - 1.

    elif mode in ['cross_entropy']:
        # 交叉熵作为奖励，衡量完整子图预测值与目标类别之间的差异
        reward = torch.log(new_subgraph_pred + EPS)[:, target_y]

    # reward += pre_reward
    reward += 0.97 * pre_reward

    return reward

*Causal Screening of an Edge Sequence

Reinforced Causal Explainer (RC-Explainer)

 主要流程框架：train_test_pool_batch3.py

def test_policy_all_with_gnd(rc_explainer, model, test_loader, topN=None):
    rc_explainer.eval()
    model.eval()

    topK_ratio_list = [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0]
    acc_count_list = np.zeros(len(topK_ratio_list))

    precision_topN_count = 0.
    recall_topN_count = 0.

    with torch.no_grad():
        for graph in iter(test_loader):
            graph = graph.to(device)
            max_budget = graph.num_edges#最大预算
            state = torch.zeros(max_budget, dtype=torch.bool)#当前状态
            # 根据 top K 比率列表计算出需要检查准确率的预算列表
            check_budget_list = [max(int(_topK * max_budget), 1) for _topK in topK_ratio_list]
            valid_budget = max(int(0.9 * max_budget), 1)#有效预算

            for budget in range(valid_budget):#每一个预算
                available_actions = state[~state].clone()#可用的动作
                # 获取下一步的动作
                _, _, make_action_id, _ = rc_explainer(graph=graph, state=state, train_flag=False)
                # 将推断的动作应用到可用动作列表中
                available_actions[make_action_id] = True
                state[~state] = available_actions.clone()#更新当前状态
                # 如果当前预算需要检查准确率
                if (budget + 1) in check_budget_list:
                    check_idx = check_budget_list.index(budget + 1)#查找当前预算在 check_budget_list 中的索引
                    subgraph = relabel_graph(graph, state)
                    # 用模型对子图进行预测
                    subgraph_pred = model(subgraph.x, subgraph.edge_index, subgraph.edge_attr, subgraph.batch)
                    # 计算准确率并累加到对应的位置
                    acc_count_list[check_idx] += sum(graph.y == subgraph_pred.argmax(dim=1))
                print('graph.ground_truth_mask[0]',graph.ground_truth_mask[0])
                # 指定了 topN & 当前预算=topN-1
                if topN is not None and budget == topN - 1:
                    print('graph.ground_truth_mask[0]',graph.ground_truth_mask[0])
                    # 累加前N个动作的精度
                    precision_topN_count += torch.sum(state*graph.ground_truth_mask[0])/topN
                    recall_topN_count += torch.sum(state*graph.ground_truth_mask[0])/sum(graph.ground_truth_mask[0])

    acc_count_list[-1] = len(test_loader)
    acc_count_list = np.array(acc_count_list)/len(test_loader)

    precision_topN_count = precision_topN_count / len(test_loader)
    recall_topN_count = recall_topN_count / len(test_loader)

    if topN is not None:
        print('\nACC-AUC: %.4f, Precision@5: %.4f, Recall@5: %.4f' %
              (acc_count_list.mean(), precision_topN_count, recall_topN_count))
    else:
        print('\nACC-AUC: %.4f' % acc_count_list.mean())
    print(acc_count_list)

    return acc_count_list.mean(), acc_count_list, precision_topN_count, recall_topN_count

 

其中这四步的实现： rc_explainer_pool.py

class RC_Explainer_Batch_star(RC_Explainer_Batch):
    def __init__(self, _model, _num_labels, _hidden_size, _use_edge_attr=False):
        super(RC_Explainer_Batch_star, self).__init__(_model, _num_labels, _hidden_size, _use_edge_attr=False)
    # 单层MLP
    def build_edge_action_prob_generator(self):
        edge_action_prob_generator = nn.ModuleList()
        for i in range(self.num_labels):
            i_explainer = Sequential(
                Linear(self.hidden_size * (2 + self.use_edge_attr), self.hidden_size * 2),
                ELU(),
                Linear(self.hidden_size * 2, self.hidden_size),
                ELU(),
                Linear(self.hidden_size, 1)
            ).to(device)
            edge_action_prob_generator.append(i_explainer)

        return edge_action_prob_generator

    def forward(self, graph, state, train_flag=False):
        #整个图表示 graph_rep-->torch.Size([64, 32])
        graph_rep = self.model.get_graph_rep(graph.x, graph.edge_index, graph.edge_attr, graph.batch)
        #若不存在已使用的边，创建全0子图表示
        if len(torch.where(state==True)[0]) == 0:
            subgraph_rep = torch.zeros(graph_rep.size()).to(device)
        else:
            subgraph = relabel_graph(graph, state)#根据状态重新标记图
            subgraph_rep = self.model.get_graph_rep(subgraph.x, subgraph.edge_index, subgraph.edge_attr, subgraph.batch)
        # 可用边索引、属性 
        ava_edge_index = graph.edge_index.T[~state].T #torch.Size([2, 3666])
        ava_edge_attr = graph.edge_attr[~state]#torch.Size([3362, 3])
        #未使用边对应的节点表示->torch.Size([2153, 32])
        ava_node_reps = self.model.get_node_reps(graph.x, ava_edge_index, ava_edge_attr, graph.batch)
        # 学习每个候选动作表示
        if self.use_edge_attr:#使用边属性信息，将未使用边嵌入可用边表示
            ava_edge_reps = self.model.edge_emb(ava_edge_attr)
            ava_action_reps = torch.cat([ava_node_reps[ava_edge_index[0]],
                                         ava_node_reps[ava_edge_index[1]],
                                         ava_edge_reps], dim=1).to(device)
        else:

            ava_action_reps = torch.cat([ava_node_reps[ava_edge_index[0]],
                                         ava_node_reps[ava_edge_index[1]]], dim=1).to(device)#torch.Size([3824, 64])
        #边动作表示生成器
        ava_action_reps = self.edge_action_rep_generator(ava_action_reps)#torch.Size([3760, 32])
        #未使用边所属图
        ava_action_batch = graph.batch[ava_edge_index[0]]#[ 0,  0,  0,  ..., 63, 63, 63] torch.Size([4016])
        #图标签
        ava_y_batch = graph.y[ava_action_batch]#[0, 0, 0,  ..., 1, 1, 1] torch.Size([3794])
        # get the unique elements in batch, in cases where some batches are out of actions.
        unique_batch, ava_action_batch = torch.unique(ava_action_batch, return_inverse=True)#[64],[3760]
        #选择一个动作，预测未使用的边的动作概率
        ava_action_probs = self.predict_star(graph_rep, subgraph_rep, ava_action_reps, ava_y_batch, ava_action_batch)
        # print(ava_action_probs,ava_action_probs.size())
        # assert len(ava_action_probs) == sum(~state)
        #每个图中最大概率及动作
        added_action_probs, added_actions = scatter_max(ava_action_probs, ava_action_batch)

        if train_flag:#训练
            rand_action_probs = torch.rand(ava_action_probs.size()).to(device)# 生成一个与未使用的边的动作概率相同大小的随机概率张量
            #每个图中最大的随机概率动作
            _, rand_actions = scatter_max(rand_action_probs, ava_action_batch)

            return ava_action_probs, ava_action_probs[rand_actions], rand_actions, unique_batch

        return ava_action_probs, added_action_probs, added_actions, unique_batch

    def predict_star(self, graph_rep, subgraph_rep, ava_action_reps, target_y, ava_action_batch):
        action_graph_reps = graph_rep - subgraph_rep#可用图表示
        action_graph_reps = action_graph_reps[ava_action_batch]#索引可用图表示
        #未使用边动作表示拼接动作图表示->完整的动作表示
        action_graph_reps = torch.cat([ava_action_reps, action_graph_reps], dim=1)

        action_probs = []
        for i_explainer in self.edge_action_prob_generator:#对于每个标签的动作解释器
            i_action_probs = i_explainer(action_graph_reps)#当前标签的动作解释器预测动作概率
            action_probs.append(i_action_probs)
        action_probs = torch.cat(action_probs, dim=1)#每个标签的动作概率连接，每一列->一个标签的动作概率
        #从预测的动作概率中索引标签对应的概率
        action_probs = action_probs.gather(1, target_y.view(-1,1))
        action_probs = action_probs.reshape(-1)#一维
        # action_probs = softmax(action_probs, ava_action_batch)
        # action_probs = F.sigmoid(action_probs)
        return action_probs

Policy Network

 论文相关代码实现：rc_explainer_pool.py  RC_Explainer_Batch_star()

ava_node_reps = self.model.get_node_reps(graph.x, ava_edge_index, ava_edge_attr, graph.batch)
        # 学习每个候选动作表示
        if self.use_edge_attr:#使用边属性信息，将未使用边嵌入可用边表示
            ava_edge_reps = self.model.edge_emb(ava_edge_attr)
            ava_action_reps = torch.cat([ava_node_reps[ava_edge_index[0]],
                                         ava_node_reps[ava_edge_index[1]],
                                         ava_edge_reps], dim=1).to(device)
        else:

            ava_action_reps = torch.cat([ava_node_reps[ava_edge_index[0]],
                                         ava_node_reps[ava_edge_index[1]]], dim=1).to(device)#torch.Size([3824, 64])
        #边动作表示生成器
        ava_action_reps = self.edge_action_rep_generator(ava_action_reps)#torch.Size([3760, 32])

论文相关代码实现：rc_explainer_pool.py 

def predict_star(self, graph_rep, subgraph_rep, ava_action_reps, target_y, ava_action_batch):
        action_graph_reps = graph_rep - subgraph_rep#可用图表示
        action_graph_reps = action_graph_reps[ava_action_batch]#索引可用图表示
        #未使用边动作表示拼接动作图表示->完整的动作表示
        action_graph_reps = torch.cat([ava_action_reps, action_graph_reps], dim=1)

        action_probs = []
        for i_explainer in self.edge_action_prob_generator:#对于每个标签的动作解释器
            i_action_probs = i_explainer(action_graph_reps)#当前标签的动作解释器预测动作概率
            action_probs.append(i_action_probs)
        action_probs = torch.cat(action_probs, dim=1)#每个标签的动作概率连接，每一列->一个标签的动作概率
        #从预测的动作概率中索引标签对应的概率
        action_probs = action_probs.gather(1, target_y.view(-1,1))
        action_probs = action_probs.reshape(-1)#一维
        # action_probs = softmax(action_probs, ava_action_batch)
        # action_probs = F.sigmoid(action_probs)
        return action_probs

 

 

Policy Gradient Training

 论文相关代码实现：train_test_pool_batch3.py  train_policy()

# 批次损失（RL REINFORCE策略梯度）
                batch_loss += torch.mean(- torch.log(beam_action_probs_list + EPS) * beam_reward_list)

Discussion

EXPERIMENTS

Evaluation Metrics

论文相关代码实现：一、ACC train_test_pool_batch3.py test_policy_all_with_gnd()

# 计算准确率并累加到对应的位置
                    acc_count_list[check_idx] += sum(graph.y == subgraph_pred.argmax(dim=1))

Evaluation of Explanations