[XLNet] Fix mems behavior (#8567)

* fix mems in xlnet

* fix use_mems

* fix use_mem_len

* fix use mems

* clean docs

* fix tf typo

* make xlnet tf for generation work

* fix tf test

* refactor use cache

* add use cache for missing models

* correct use_cache in generate

* correct use cache in tf generate

* fix tf

* correct getattr typo

* make sylvain happy

* change in docs as well

* do not apply to cookie cutter statements

* fix tf test

* make pytorch model fully backward compatible

src/transformers/models/bert/modeling_bert.py

@@ -888,7 +888,7 @@ class BertModel(BertPreTrainedModel):
 
 @add_start_docstrings(
     """
-    Bert Model with two heads on top as done during the pre-training: a `masked language modeling` head and a `next
+    Bert Model with two heads on top as done during the pretraining: a `masked language modeling` head and a `next
     sentence prediction (classification)` head.
     """,
     BERT_START_DOCSTRING,

src/transformers/models/bert/modeling_tf_bert.py

@@ -90,7 +90,7 @@ TF_BERT_PRETRAINED_MODEL_ARCHIVE_LIST = [
 
 class TFBertPreTrainingLoss:
     """
-    Loss function suitable for BERT-like pre-training, that is, the task of pretraining a language model by combining
+    Loss function suitable for BERT-like pretraining, that is, the task of pretraining a language model by combining
     NSP + MLM. .. note:: Any label of -100 will be ignored (along with the corresponding logits) in the loss
     computation.
     """
@@ -878,7 +878,7 @@ class TFBertModel(TFBertPreTrainedModel):
 
 @add_start_docstrings(
     """
-Bert Model with two heads on top as done during the pre-training:
+Bert Model with two heads on top as done during the pretraining:
     a `masked language modeling` head and a `next sentence prediction (classification)` head.
     """,
     BERT_START_DOCSTRING,