PretrainedTokenizer cleanup: Typehints, decode_batch
This is very very minor cleanup

- adds `decode_batch` which calls `.decode` on every entry in a list.
- A few cosmetic changes to tokenization_utils.py (type hints using defaultdict)
- adds type hints in files I touched.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3837?src=pr&el=h1) Report
> Merging [#3837](https://codecov.io/gh/huggingface/transformers/pull/3837?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f399c00610506325bc1690f0e68c6885e73395ec&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `84.21%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3837/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3837?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3837      +/-   ##
==========================================
- Coverage   78.48%   78.48%   -0.01%     
==========================================
  Files         106      106              
  Lines       17930    17934       +4     
==========================================
+ Hits        14072    14075       +3     
- Misses       3858     3859       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3837?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3837/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `87.50% <82.35%> (-0.07%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/3837/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtLnB5) | `83.46% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3837/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.76% <0.00%> (ø)` | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3837?src=pr&el=footer). Last update [f399c00...41502fa](https://codecov.io/gh/huggingface/transformers/pull/3837?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
