Fix imports for LSG code: explicit BERT import + import math + logger

modeling_lsg_bert.py

@@ -1,6 +1,5 @@
 from logging import warn
 from transformers.models.bert.modeling_bert import (
-    BertPreTrainedModel,
     BertEmbeddings,
     BertModel,
     BertForPreTraining,
@@ -10,10 +9,11 @@ from transformers.models.bert.modeling_bert import (
     BertForMultipleChoice,
     BertForTokenClassification,
     BertForQuestionAnswering,
-    BertSelfOutput,
-    BertPooler,
+    BertPreTrainedModel,
     BertEncoder,
     BertLayer,
+    BertPooler,
+    BertSelfOutput,
 )
 import torch
 import torch.nn as nn