Moving fill-mask pipeline to new testing scheme (#12943)

* Fill mask pipelines test updates.

* Model eval !!

* Adding slow test with actual values.

* Making all tests pass (skipping quite a bit.)

* Doc styling.

* Better doc cleanup.

* Making an explicit test with no pad token tokenizer.

* Typo.

tests/test_modeling_reformer.py

@@ -189,6 +189,7 @@ class ReformerModelTester:
     def get_pipeline_config(self):
         config = self.get_config()
         config.vocab_size = 100
+        config.is_decoder = False
         return config
 
     def create_and_check_reformer_model(self, config, input_ids, input_mask, choice_labels):