Fix bug in max_seq_length for preprocessing in ner example
**Summary**

If you try to use a value for `max_seq_length` that is less than 128 in the NER example, the maximum sequence length is exceeded when the predictions are made. There is a warning logged for this "Maximum sequence length exceeded: No prediction for.." and predictions cannot be made for these tokens.

**Changes**

Two changes are made:

- In `preprocess.py`, `subword_len_counter` is set to `current_subwords_len` when a blank line is inserted to split up sequences that exceed the maximum sequence length.
- `tokenizer.num_added_tokens()` is subtracted from `max_len` to account for the additional tokens inserted by the BERT tokenizer.


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3803?src=pr&el=h1) Report
> Merging [#3803](https://codecov.io/gh/huggingface/transformers/pull/3803?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/01c37dcdb529bff38aadd51001cb5812e5fe9b21&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3803/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3803?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3803   +/-   ##
=======================================
  Coverage   78.26%   78.27%           
=======================================
  Files         106      106           
  Lines       17964    17964           
=======================================
+ Hits        14060    14061    +1     
+ Misses       3904     3903    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3803?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3803/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.84% <0.00%> (+0.16%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3803?src=pr&el=footer). Last update [01c37dc...8f77ccd](https://codecov.io/gh/huggingface/transformers/pull/3803?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>You might want to open this PR on https://github.com/stefan-it/fine-tuned-berts-seq instead, as the script is hosted there right now (cc @stefan-it)<|||||>> You might want to open this PR on https://github.com/stefan-it/fine-tuned-berts-seq instead, as the script is hosted there right now (cc @stefan-it)

I thought it would be easier just to have the script in the example, but can close this pr and open it there if you prefer?<|||||>What do you think @stefan-it? Are you ok with us including the script here?<|||||>Hi, sorry for the late reply! I've fixed some errors in the script last week. Would be great if @r-tinn could check the latest Version! If it's working then you can of course integrate it into Transformers :)<|||||>Looks good to me, the problem seems to be fixed<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
