[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/ctrl/configuration_ctrl.py

@@ -61,6 +61,9 @@ class CTRLConfig(PretrainedConfig):
             The epsilon to use in the layer normalization layers
         initializer_range (:obj:`float`, `optional`, defaults to 0.02):
             The standard deviation of the truncated_normal_initializer for initializing all weight matrices.
+        use_cache (:obj:`bool`, `optional`, defaults to :obj:`True`):
+            Whether or not the model should return the last key/values attentions (not used by all models).
+
 
     Examples::
 
@@ -98,6 +101,7 @@ class CTRLConfig(PretrainedConfig):
         summary_activation=None,
         summary_proj_to_labels=True,
         summary_first_dropout=0.1,
+        use_cache=True,
         **kwargs
     ):
         super().__init__(**kwargs)
@@ -119,6 +123,7 @@ class CTRLConfig(PretrainedConfig):
         self.summary_activation = summary_activation
         self.summary_first_dropout = summary_first_dropout
         self.summary_proj_to_labels = summary_proj_to_labels
+        self.use_cache = use_cache
 
     @property
     def max_position_embeddings(self):