[Tokenization] Fix #5181 - make #5155 more explicit - move back the default logging level in tests to WARNING (#5252)

* fix-5181

Padding to max sequence length while truncation to another length was wrong on slow tokenizers

* clean up and fix #5155

* fix XLM test

* Fix tests for Transfo-XL

* logging only above WARNING in tests

* switch slow tokenizers tests in @slow

* fix Marian truncation tokenization test

* style and quality

* make the test a lot faster by limiting the sequence length used in tests

src/transformers/tokenization_utils_base.py

@@ -1890,7 +1890,7 @@ class PreTrainedTokenizerBase(SpecialTokensMixin):
         if return_attention_mask is None:
             return_attention_mask = "attention_mask" in self.model_input_names
 
-        if padding_strategy == PaddingStrategy.LONGEST and max_length is None:
+        if padding_strategy == PaddingStrategy.LONGEST:
             max_length = len(encoded_inputs["input_ids"])
 
         needs_to_be_padded = (