change the defaut optimizer to l2 cddual in EUR-lex config

Author: ericliu8168

example_config/EUR-Lex/tree_l2svm.yml

@@ -6,7 +6,7 @@ data_name: EUR-Lex
 # train
 seed: 1337
 linear: true
-liblinear_options: "-s 2 -B 1 -e 0.0001 -q"
+liblinear_options: "-s 1 -B 1 -e 0.0001 -q"
 linear_technique: tree
 
 # eval

libmultilabel/linear/tree.py

@@ -225,7 +225,8 @@ def collect_stat(node: Node):
     root.dfs(collect_stat)
 
     # 16 is because when storing sparse matrices, indices (int64) require 8 bytes and floats require 8 bytes
-    return total_num_weights * 16
+    # 2/3 is because that dual coordinate descent method can further get a sparse model vector to save space
+    return total_num_weights * 16 * 2/3
 
 
 def _train_node(y: sparse.csr_matrix, x: sparse.csr_matrix, options: str, node: Node):