Force pad_token_id to be set before padding for standard tokenizer
I think batch_encode_plus with a proper padding strategy should not be allowed if the pad_token_id is not set. I don't feel like it helps the user to have a python list of lists with None values that he can't transform to a torch.Tensor anyways. 

As a remedy I think it is alright if a new pad_token is added or whether it is set to an existing special_token. 

This behavior is already enforced for FastTokenizer, so the PR should also make it easier to transition from Tokenizer to FastTokenizer. 

I will fix the tests and add a new one if you guys agree. 

@mfuntowicz @thomwolf @LysandreJik 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3035?src=pr&el=h1) Report
> Merging [#3035](https://codecov.io/gh/huggingface/transformers/pull/3035?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/298bed16a841fae3608d334441ccae4d9043611f?src=pr&el=desc) will **decrease** coverage by `1.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3035/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3035?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3035      +/-   ##
==========================================
- Coverage   77.18%   76.16%   -1.03%     
==========================================
  Files          98       98              
  Lines       16063    16065       +2     
==========================================
- Hits        12399    12236     -163     
- Misses       3664     3829     +165
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3035?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `91.72% <100%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/3035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.71% <0%> (-10%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/3035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.48% <0%> (-2.3%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `96.03% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/3035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <0%> (-1.35%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3035?src=pr&el=footer). Last update [298bed1...2879683](https://codecov.io/gh/huggingface/transformers/pull/3035?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Is good to merge for me after checking adapted tests in tokenization_utils.py @LysandreJik 