Make the order of additional special tokens deterministic
In `SpecialTokensMixin.all_special_tokens_extended`, deduplication is performed by `all_toks = list(set(all_toks))`. However, this will change the ordering of additional special tokens, and the order depends on the hash seed of set data structure. This will result in non-deterministic id of additional special tokens added to `AutoTokenizer.from_pretrained` method. Therefore, I changed this problematic line to `all_toks = list(OrderedDict.fromkeys(all_toks))`. This line will deduplicate `all_toks` while still keeping the original ordering.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5704?src=pr&el=h1) Report
> Merging [#5704](https://codecov.io/gh/huggingface/transformers/pull/5704?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0befb513278f6e42b722be340dbc667e0ba2718e&el=desc) will **decrease** coverage by `0.94%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5704/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5704?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5704      +/-   ##
==========================================
- Coverage   78.26%   77.32%   -0.95%     
==========================================
  Files         146      146              
  Lines       25998    25998              
==========================================
- Hits        20348    20102     -246     
- Misses       5650     5896     +246     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5704?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5704/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.60% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5704/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.38% <0.00%> (-73.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5704/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.53% <0.00%> (+69.51%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5704?src=pr&el=footer). Last update [0befb51...c706cc9](https://codecov.io/gh/huggingface/transformers/pull/5704?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
