[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/generation_tf_utils.py

@@ -38,6 +38,7 @@ class TFGenerationMixin:
 
     def _use_cache(self, outputs, use_cache):
         """During generation, decide whether to pass the `past` variable to the next forward pass."""
+        use_cache = getattr(self.config, "use_cache", False)
         if len(outputs) <= 1 or use_cache is False:
             return False
         if hasattr(self.config, "mem_len") and self.config.mem_len == 0:
@@ -194,7 +195,6 @@ class TFGenerationMixin:
         min_length = min_length if min_length is not None else self.config.min_length
         do_sample = do_sample if do_sample is not None else self.config.do_sample
         early_stopping = early_stopping if early_stopping is not None else self.config.early_stopping
-        use_cache = use_cache if use_cache is not None else self.config.use_cache
         num_beams = num_beams if num_beams is not None else self.config.num_beams
         temperature = temperature if temperature is not None else self.config.temperature
         top_k = top_k if top_k is not None else self.config.top_k
@@ -224,7 +224,6 @@ class TFGenerationMixin:
         assert isinstance(min_length, int) and min_length >= 0, "`min_length` should be a positive integer."
         assert isinstance(do_sample, bool), "`do_sample` should be a boolean."
         assert isinstance(early_stopping, bool), "`early_stopping` should be a boolean."
-        assert isinstance(use_cache, bool), "`use_cache` should be a boolean."
         assert isinstance(num_beams, int) and num_beams > 0, "`num_beams` should be a strictly positive integer."
         assert temperature > 0, "`temperature` should be strictly positive."
         assert isinstance(top_k, int) and top_k >= 0, "`top_k` should be a positive integer."