作用：对 top输出到多个 Layer的情况进行分割，建立完整的网络结构
重要的参数说明举例：
- layer_idx_to_layer_name[i]
记录各层的名称，如 [0x00000000] "input" - blob_name_to_last_top_idx[“conv1”]=(1,0)
这个例子相当于说”conv1” 这层是第1层的第0个top， - bottom_idx_to_source_top_idx[bottom_idx=(2,0)] = (1,0);
相当于说：第2层，第0个bottom，对应着第1层，第0个top
即： [(0x00000001, 0x00000000)] (0x00000000, 0x00000000) - top_idx_to_bottom_count[(1,0)]=2
表示第1个layer的第0个top有2个top(blob?)，要分叉，程序会对此建立新的分叉层
理解：整个函数分为两个部分
1. 遍历整个网络，记录每一个Layer的top的使用情况，记录结构放在 top_idx_to_bottom_count中。
2. 遍历整个网络，对 top_idx_to_bottom_count > 1 的情况进行处理: 分两步走
第一步. 首先是对有多个top层的Layer进行分割，主要的做法是在该层的后面新建一个Layer ，这个新的Layer的会按照 top_idx_to_bottom_count 的个数和约定的分割名称（SplitBlobName）去新建top，参考void ConfigureSplitLayer…
第二步，是对使用同一个top的后续层的bottom的blob进行改名（使用与上一步相同的命名规则）。
下面是源码
void InsertSplits(const NetParameter& param, NetParameter* param_split) {
// Initialize by copying from the input NetParameter.
param_split->CopyFrom(param);
param_split->clear_layer();
map<string, pair<int, int> > blob_name_to_last_top_idx;
map<pair<int, int>, pair<int, int> > bottom_idx_to_source_top_idx;
map<pair<int, int>, int> top_idx_to_bottom_count;
map<pair<int, int>, float> top_idx_to_loss_weight;
map<pair<int, int>, int> top_idx_to_bottom_split_idx;
map<int, string> layer_idx_to_layer_name; // 每层网络的名称
for (int i = 0; i < param.layer_size(); ++i) {
const LayerParameter& layer_param = param.layer(i);
layer_idx_to_layer_name[i] = layer_param.name();
for (int j = 0; j < layer_param.bottom_size(); ++j) {
const string& blob_name = layer_param.bottom(j);
if (blob_name_to_last_top_idx.find(blob_name) ==
blob_name_to_last_top_idx.end()) {
LOG(FATAL) << "Unknown bottom blob '" << blob_name << "' (layer '"
<< layer_param.name() << "', bottom index " << j << ")";
}
const pair<int, int>& bottom_idx = make_pair(i, j);
const pair<int, int>& top_idx = blob_name_to_last_top_idx[blob_name];
bottom_idx_to_source_top_idx[bottom_idx] = top_idx;
++top_idx_to_bottom_count[top_idx];
}
for (int j = 0; j < layer_param.top_size(); ++j) {
const string& blob_name = layer_param.top(j);
blob_name_to_last_top_idx[blob_name] = make_pair(i, j);
}
// A use of a top blob as a loss should be handled similarly to the use of
// a top blob as a bottom blob to another layer.
const int last_loss =
std::min(layer_param.loss_weight_size(), layer_param.top_size());
for (int j = 0; j < last_loss; ++j) {
const string& blob_name = layer_param.top(j);
const pair<int, int>& top_idx = blob_name_to_last_top_idx[blob_name];
top_idx_to_loss_weight[top_idx] = layer_param.loss_weight(j);
if (top_idx_to_loss_weight[top_idx]) {
++top_idx_to_bottom_count[top_idx];
}
}
}
for (int i = 0; i < param.layer_size(); ++i) {
LayerParameter* layer_param = param_split->add_layer();
layer_param->CopyFrom(param.layer(i));
// Replace any shared bottom blobs with split layer outputs.
for (int j = 0; j < layer_param->bottom_size(); ++j) {
const pair<int, int>& top_idx =
bottom_idx_to_source_top_idx[make_pair(i, j)];
//表示第i层的第j个bottom, 是第top_idx0层的第top_idx1个top,
// 有split_count个bottom
const int split_count = top_idx_to_bottom_count[top_idx];
if (split_count > 1) {
const string& layer_name = layer_idx_to_layer_name[top_idx.first];
const string& blob_name = layer_param→bottom(j);
// 只是改个名称,这个名称由 SplitBlobName创建
layer_param->set_bottom(j, SplitBlobName(layer_name,
blob_name, top_idx.second, top_idx_to_bottom_split_idx[top_idx]++));
}
}
// Create split layer for any top blobs used by other layer as bottom
// blobs more than once.
for (int j = 0; j < layer_param->top_size(); ++j) {
const pair<int, int>& top_idx = make_pair(i, j);
//表示第i层的第j个top有split_count个blob
const int split_count = top_idx_to_bottom_count[top_idx];
if (split_count > 1) {
// 如果要分叉（对应多个top）
const string& layer_name = layer_idx_to_layer_name[i]; //第i层的名称
const string& blob_name = layer_param->top(j); // 第j个top的blob名称
LayerParameter* split_layer_param = param_split->add_layer(); // 新建一层
const float loss_weight = top_idx_to_loss_weight[top_idx];
// 第i层的第j个top有的loss_weight
ConfigureSplitLayer(layer_name, blob_name, j, split_count,
loss_weight, split_layer_param);
if (loss_weight) {
layer_param->clear_loss_weight();
top_idx_to_bottom_split_idx[top_idx]++;
}
}
}
}
}