Remove unneeded call convert_ids_to_tokens.
Author: tshauck
Pull request: yes

On the base `PreTrainedTokenizer` object, calling `convert_tokens_to_string` returns an error because the method tries to call `convert_ids_to_tokens.` however the input should already be a list of tokens not ids.

I noticed this when creating a tokenizer that inherits from the Pretrained Tokenizer, and trying to test it with:

```python
assert transformers_tokenizer.decode([1, 15, 22, 15, 2]) == ">MVM<"
```

which eventually results in:

```python
self = <gcgc.third_party.GCGCTransformersTokenizer object at 0x16539ead0>, ids = ['>', 'M', 'V', 'M', '<'], skip_special_tokens = False

    def convert_ids_to_tokens(
        self, ids: Union[int, List[int]], skip_special_tokens: bool = False
    ) -> Union[int, List[int]]:
        """ Converts a single index or a sequence of indices (integers) in a token "
            (resp.) a sequence of tokens (str), using the vocabulary and added tokens.

            Args:
                skip_special_tokens: Don't decode special tokens (self.all_special_tokens). Default: False
        """
        if isinstance(ids, int):
            if ids in self.added_tokens_decoder:
                return self.added_tokens_decoder[ids]
            else:
                return self._convert_id_to_token(ids)
        tokens = []
        for index in ids:
>           index = int(index)
E           ValueError: invalid literal for int() with base 10: '>'
```

But I don't think this should get called in the first place?

I think I'm actually go override this now that I understand the process better, but this still seemed incorrect.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4836?src=pr&el=h1) Report
> Merging [#4836](https://codecov.io/gh/huggingface/transformers/pull/4836?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ac921f0385616be40adbbd5302d7f58d5c976ca8&el=desc) will **decrease** coverage by `1.12%`.
> The diff coverage is `100.00%`.

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```diff
@@            Coverage Diff             @@
##           master    #4836      +/-   ##
==========================================
- Coverage   78.45%   77.32%   -1.13%     
==========================================
  Files         146      146              
  Lines       26047    26047              
==========================================
- Hits        20434    20142     -292     
- Misses       5613     5905     +292     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4836?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4836/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.36% <100.00%> (+0.40%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/4836/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.38% <0.00%> (-73.39%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4836/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/4836/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |

------

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