Testing that batch_encode_plus is the same as encode_plus (#2973)

* Testing that encode_plus and batch_encode_plus behave the same way

Spoiler alert: they don't

* Testing rest of arguments in batch_encode_plus

* Test tensor return in batch_encode_plus

* Addressing Sam's comments

* flake8

* Simplified with `num_added_tokens`

src/transformers/tokenization_t5.py

@@ -98,6 +98,12 @@ class T5Tokenizer(PreTrainedTokenizer):
             additional_special_tokens=additional_special_tokens,
             **kwargs,
         )
+        self.max_len_single_sentence = (
+            self.max_len
+        )  # no default special tokens - you can update this value if you add special tokens
+        self.max_len_sentences_pair = (
+            self.max_len
+        )  # no default special tokens - you can update this value if you add special tokens
 
         try:
             import sentencepiece as spm