Wrong logic in `batch_encode_plus()`
# 🐛 Bug

I'm trying to use `batch_encode_plus()` with already tokenized ID as input.

So I have a `list of list of int` as input. For example :

```python
x = [[0, 83, 313, 11, 10551, 2278, 16, 2183, 1958, 804, 7, 916, 11, 13933, 982, 4, 286, 68, 5046, 6, 37, 40, 3627, 231, 2697, 9, 1958, 11, 41, 36027, 27894, 1001, 21466, 424, 2233, 4, 2], [0, 20, 291, 212, 13989, 191, 3772, 42, 983, 4, 815, 34, 1714, 8617, 187, 63, 17692, 11, 8008, 4, 993, 864, 549, 1492, 2624, 5391, 9686, 8, 12291, 240, 7, 464, 4, 2]]
```

When I call `batch_encode_plus()` with this input, I receive `AssertionError` :

```
tokenizer.batch_encode_plus(x)
# >>> line 1130, in batch_encode_plus
#                    assert len(ids_or_pair_ids) == 2
```

---

After reviewing the code, it seems there is an error in the logic. Here :

https://github.com/huggingface/transformers/blob/b370cc7e99c5b8c7436154d4694c33b461ea0f08/src/transformers/tokenization_utils.py#L1128-L1137

since I gave only tokenized ID and not a tuple of tokenized ID (not a pair), the logic should bring me to the `else` clause. But instead, I enter the `if` clause, leading to the assertion error.

**It's currently not possible to use `batch_encode_plus()` with already tokenized text / IDs** if the input is not a pair.

---

I think this line :

https://github.com/huggingface/transformers/blob/b370cc7e99c5b8c7436154d4694c33b461ea0f08/src/transformers/tokenization_utils.py#L1129

Should be changed to this :

```python
if isinstance(ids_or_pair_ids, (list, tuple)) and len(ids_or_pair_ids) == 2:
```
