Unexpected result when tokenizing a single token rather than a sentence with multilingual tokenizer
I want to check how a word is divided into sub-words, but I found an unexpected result.

e.g. mbart:
```python
mbart_tokenizer = AutoTokenizer.from_pretrained('facebook/mbart-large-cc25', use_fast=True)
mbart_tokenizer.tokenize('.')
>>> ['▁', '.']
mbart_tokenizer.tokenize('a dog.')
>>> ['▁a', '▁dog', '.']
```
Why the `'.'` is tokenized to two sub-words `['▁', '.']` when it is a single token but to `'.'` when it is a part of a sentence?

e.g. mT5
```python
t5_tokenizer = T5Tokenizer.from_pretrained("google/mt5-small")
t5_tokenizer.tokenize('.')
>>> ['▁', '.']
t5_tokenizer.tokenize('a')
>>> ['▁', 'a']
mbart_tokenizer.tokenize('a dog.')
>>> ['▁', 'a', '▁dog', '.']
```
