Fix convert_tokens_to_string when decoder is None
Author: dszeto
Pull request: yes

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In `convert_tokens_to_string` of `src/transformers/tokenization_utils_fast.py`, `self.backend_tokenizer.decoder` can be `None` when the tokenizer is trained by `tokenizers.trainers.WordLevelTrainer`. This fix adds a check and falls back to joining tokens with a space when no decoder is found. This follows the default behavior in the Rust implementation of Tokenizers.

Original question posted on https://discuss.huggingface.co/t/pretrainedtokenizerfast-convert-tokens-to-string-always-assumes-the-presence-of-decoder/114978

All existing tokenizer tests are passing. Style changes were made by `make fixup`.

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Comments:
I want to emphasize my support for this PR with a simply reproducible error (and a real use-case) that this PR fixes:
When running vLLM with huggingface models which have a `PreTrainedTokenizerFast` tokenizer, for example:
```shell
python3 -m vllm.entrypoints.openai.api_server --model jounce/dummy-phi3
```
Currently fails like so:
```shell
ERROR 11-11 09:00:29 engine.py:158]   File "/usr/local/lib/python3.12/dist-packages/vllm/transformers_utils/detokenizer.py", line 122, in decode_sequence_inplace
ERROR 11-11 09:00:29 engine.py:158]     read_offset) = detokenize_incrementally(
ERROR 11-11 09:00:29 engine.py:158]                    ^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 11-11 09:00:29 engine.py:158]   File "/usr/local/lib/python3.12/dist-packages/vllm/transformers_utils/detokenizer.py", line 301, in detokenize_incrementally
ERROR 11-11 09:00:29 engine.py:158]     prefix_text = tokenizer.convert_tokens_to_string(
ERROR 11-11 09:00:29 engine.py:158]                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 11-11 09:00:29 engine.py:158]   File "/usr/local/lib/python3.12/dist-packages/transformers/tokenization_utils_fast.py", line 641, in convert_tokens_to_string
ERROR 11-11 09:00:29 engine.py:158]     return self.backend_tokenizer.decoder.decode(tokens)
ERROR 11-11 09:00:29 engine.py:158]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 11-11 09:00:29 engine.py:158] AttributeError: 'NoneType' object has no attribute 'decode'
```

Great PR, tnx :)

Hey @ArthurZucker , would be awesome if you could take a quick look at this.

Please let me know if someone else should review.

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