fix_ parallel_tree_learner_split_info

#6491 (comment)
This code is used for computing the buffer size of the communicating the split info the in distributed training. But lost the two most recently added parameters(right_sum_gradient_and_hessian, left_sum_gradient_and_hessian) size.
When use parallel_tree_learner method with number of categories of one feature larger than 28 (default max_cat_to_onehot(32)-2*2), it will resut in an Segmentation fault.

src/treelearner/split_info.hpp

@@ -53,7 +53,7 @@ struct SplitInfo {
   bool default_left = true;
   int8_t monotone_type = 0;
   inline static int Size(int max_cat_threshold) {
-    return 2 * sizeof(int) + sizeof(uint32_t) + sizeof(bool) + sizeof(double) * 7 + sizeof(data_size_t) * 2 + max_cat_threshold * sizeof(uint32_t) + sizeof(int8_t);
+    return 2 * sizeof(int) + sizeof(uint32_t) + sizeof(bool) + sizeof(double) * 7 + sizeof(data_size_t) * 2 + max_cat_threshold * sizeof(uint32_t) + sizeof(int8_t) + sizeof(int64_t)*2;
   }
 
   inline void CopyTo(char* buffer) const {