Test model outputs equivalence (#6445)

* Test model outputs equivalence

* Fix failing tests

* From dict to kwargs

* DistilBERT

* Addressing @sgugger and @patrickvonplaten's comments

tests/test_modeling_t5.py

@@ -37,6 +37,8 @@ class T5ModelTester:
         self.batch_size = 13
         self.encoder_seq_length = 7
         self.decoder_seq_length = 9
+        # For common tests
+        self.seq_length = self.decoder_seq_length
         self.is_training = True
         self.use_attention_mask = True
         self.use_labels = True