unify BERT config

Author: donglihe-hub

example_config/EUR-Lex-57k/bert_lwan_tune.yml

@@ -4,7 +4,7 @@ val_file: data/EUR-Lex-57k/valid.txt
 test_file: data/EUR-Lex-57k/test.txt
 data_name: EUR-Lex-57k
 min_vocab_freq: 1
-max_seq_length: 500
+max_seq_length: 512
 include_test_labels: true
 remove_no_label_data: false
 add_special_tokens: false