GPT2 tokenizer is so slow because of sum()
## 🐛 Bug
Hi,
As the discussion started in that  #1621 issue, GPT2 tokenization is so slow even with 50MB of the dataset.

I'm using `run_lm_finetuning.py` and here are the steps to reproduce the problem:

- Have a dataset not an even bigger one. 20MB of a dataset is enough.
- Call `run_lm_finetuning.py` to train (finetune) the dataset. Here are my parameters:
```
--train_data_file "/train/datafile" \
--eval_data_file "/eval/datafile" \
--output_dir "/train/model" \
--model_type gpt2 \
--model_name_or_path distilgpt2 \
--cache_dir "/train/cache" \
--do_train \
--evaluate_during_training \
--per_gpu_train_batch_size 1 \
--per_gpu_eval_batch_size 1 \
--gradient_accumulation_steps 5 \
--overwrite_output_dir \
--seed 99
```
- You'll see it'll spend 20+ mins (depends on your cpu) to tokenize just 50MB of a text file.

I dug into `huggingface/transformers` 's codebase and profiled the tokenization process. And it is obvious that this summation drains the time:
https://github.com/huggingface/transformers/blob/155c782a2ccd103cf63ad48a2becd7c76a7d2115/transformers/tokenization_utils.py#L644

I run profiler and here is the result:

```
73791524 function calls in 1566.379 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    27157    0.083    0.000    0.109    0.000 <frozen importlib._bootstrap>:1009(_handle_fromlist)
    27157    0.065    0.000    0.128    0.000 _bootlocale.py:33(getpreferredencoding)
    81471    0.070    0.000    0.327    0.000 locale.py:589(setlocale)
    27157    0.422    0.000    0.876    0.000 locale.py:647(getpreferredencoding)
    27157    0.363    0.000    5.997    0.000 regex.py:328(findall)
    27157    0.662    0.000    1.682    0.000 regex.py:434(_compile)
  4815114    8.744    0.000   16.641    0.000 tokenization_gpt2.py:139(bpe)
  2030532    1.116    0.000    1.887    0.000 tokenization_gpt2.py:149(<lambda>)
    27157   22.702    0.001  110.038    0.004 tokenization_gpt2.py:180(_tokenize)
 25459527    5.702    0.000    5.702    0.000 tokenization_gpt2.py:194(<genexpr>)
 10242602    1.764    0.000    1.764    0.000 tokenization_gpt2.py:195(<genexpr>)
  1377876    1.678    0.000    1.975    0.000 tokenization_gpt2.py:91(get_pairs)
    95205    0.526    0.000    0.910    0.000 tokenization_utils.py:1043(special_tokens_map)
    95205    0.932    0.000    1.987    0.000 tokenization_utils.py:1055(all_special_tokens)
        1    0.119    0.119 1566.379 1566.379 tokenization_utils.py:615(tokenize)
    40789    0.099    0.000    0.169    0.000 tokenization_utils.py:623(split_on_token)
        1    0.287    0.287 1566.260 1566.260 tokenization_utils.py:641(split_on_tokens)
    54417    0.698    0.000  112.123    0.002 tokenization_utils.py:659(<genexpr>)
    27157    0.063    0.000    0.063    0.000 {built-in method _locale.nl_langinfo}
    81471    0.252    0.000    0.252    0.000 {built-in method _locale.setlocale}
   761640    0.384    0.000    0.384    0.000 {built-in method builtins.getattr}
    54314    0.022    0.000    0.022    0.000 {built-in method builtins.hasattr}
   516605    0.150    0.000    0.150    0.000 {built-in method builtins.isinstance}
  1821447    0.159    0.000    0.159    0.000 {built-in method builtins.len}
   472563    3.469    0.000    5.355    0.000 {built-in method builtins.min}
        1 1453.081 1453.081 1565.204 1565.204 {built-in method builtins.sum}
  2043214    0.297    0.000    0.297    0.000 {method 'add' of 'set' objects}
   456488    0.055    0.000    0.055    0.000 {method 'append' of 'list' objects}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
  4815114    1.169    0.000    1.169    0.000 {method 'encode' of 'str' objects}
  5550977   16.572    0.000   18.336    0.000 {method 'extend' of 'list' objects}
    27157    3.952    0.000    3.952    0.000 {method 'findall' of '_regex.Pattern' objects}
  2057689    0.784    0.000    0.784    0.000 {method 'get' of 'dict' objects}
   735863    0.233    0.000    0.233    0.000 {method 'index' of 'tuple' objects}
  4894984   38.307    0.000   44.010    0.000 {method 'join' of 'str' objects}
        1    0.000    0.000    0.000    0.000 {method 'keys' of 'dict' objects}
  4855903    1.365    0.000    1.365    0.000 {method 'split' of 'str' objects}
    68048    0.009    0.000    0.009    0.000 {method 'strip' of 'str' objects}
    95205    0.024    0.000    0.024    0.000 {method 'values' of 'dict' objects}

```
I turned it into this by removing `sum()`
```
(self._tokenize(token, **kwargs) if token not \
                    in self.added_tokens_encoder and token not in self.all_special_tokens \
                    else [token] for token in tokenized_text)
```
and here is the profiler result:

```
   73275678 function calls in 121.030 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    27157    0.058    0.000    0.076    0.000 <frozen importlib._bootstrap>:1009(_handle_fromlist)
    27157    0.041    0.000    0.084    0.000 _bootlocale.py:33(getpreferredencoding)
    81471    0.058    0.000    0.211    0.000 locale.py:589(setlocale)
    27157    0.330    0.000    0.625    0.000 locale.py:647(getpreferredencoding)
    27157    0.267    0.000    4.996    0.000 regex.py:328(findall)
    27157    0.434    0.000    1.160    0.000 regex.py:434(_compile)
  4815114    9.797    0.000   18.875    0.000 tokenization_gpt2.py:139(bpe)
  2030532    1.270    0.000    2.100    0.000 tokenization_gpt2.py:149(<lambda>)
    27157   24.693    0.001  119.272    0.004 tokenization_gpt2.py:180(_tokenize)
 25459527    6.204    0.000    6.204    0.000 tokenization_gpt2.py:194(<genexpr>)
 10242602    1.975    0.000    1.975    0.000 tokenization_gpt2.py:195(<genexpr>)
  1377876    2.002    0.000    2.328    0.000 tokenization_gpt2.py:91(get_pairs)
    68050    0.287    0.000    0.475    0.000 tokenization_utils.py:1043(special_tokens_map)
    68050    0.507    0.000    1.061    0.000 tokenization_utils.py:1055(all_special_tokens)
        1    0.031    0.031  121.030  121.030 tokenization_utils.py:615(tokenize)
    27263    0.077    0.000    0.158    0.000 tokenization_utils.py:623(split_on_token)
        1    0.178    0.178  120.999  120.999 tokenization_utils.py:641(split_on_tokens)
        1    0.330    0.330  120.350  120.350 tokenization_utils.py:659(<listcomp>)
    27157    0.043    0.000    0.043    0.000 {built-in method _locale.nl_langinfo}
    81471    0.148    0.000    0.148    0.000 {built-in method _locale.setlocale}
   544400    0.188    0.000    0.188    0.000 {built-in method builtins.getattr}
    54314    0.014    0.000    0.014    0.000 {built-in method builtins.hasattr}
   407985    0.092    0.000    0.092    0.000 {built-in method builtins.isinstance}
  1807921    0.181    0.000    0.181    0.000 {built-in method builtins.len}
   472563    3.992    0.000    6.092    0.000 {built-in method builtins.min}
  2043214    0.326    0.000    0.326    0.000 {method 'add' of 'set' objects}
   456488    0.064    0.000    0.064    0.000 {method 'append' of 'list' objects}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
  4815114    1.259    0.000    1.259    0.000 {method 'encode' of 'str' objects}
  5550977   18.064    0.000   20.040    0.000 {method 'extend' of 'list' objects}
    27157    3.569    0.000    3.569    0.000 {method 'findall' of '_regex.Pattern' objects}
  2057689    0.839    0.000    0.839    0.000 {method 'get' of 'dict' objects}
   735863    0.273    0.000    0.273    0.000 {method 'index' of 'tuple' objects}
  4894984   41.821    0.000   48.026    0.000 {method 'join' of 'str' objects}
        1    0.000    0.000    0.000    0.000 {method 'keys' of 'dict' objects}
  4842377    1.597    0.000    1.597    0.000 {method 'split' of 'str' objects}
    54522    0.007    0.000    0.007    0.000 {method 'strip' of 'str' objects}
    68050    0.012    0.000    0.012    0.000 {method 'values' of 'dict' objects}
```

You can see 121 seconds vs 1566 seconds.  It is 12x times faster without `sum()`. Okay lets discuss do we need `sum()`? Actually, not. Because the `sum()` just flattens the array with the leanest way and there are far more efficient ways. See that [answer](https://stackoverflow.com/a/953097) on StackOverflow.
Also as written in official python [doc](https://docs.python.org/3/library/functions.html#sum) , `sum()` is developed for numbers rather than strings.

So I replaced `sum()` with `list(itertools.chain.from_iterable(text))` as follows and run profiler.

```
return list(itertools.chain.from_iterable((self._tokenize(token, **kwargs) if token not \
                    in self.added_tokens_encoder and token not in self.all_special_tokens \
                    else [token] for token in tokenized_text)))
```
Here is the result:

```
         73791524 function calls in 114.720 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    27157    0.045    0.000    0.060    0.000 <frozen importlib._bootstrap>:1009(_handle_fromlist)
    27157    0.035    0.000    0.067    0.000 _bootlocale.py:33(getpreferredencoding)
    81471    0.045    0.000    0.159    0.000 locale.py:589(setlocale)
    27157    0.277    0.000    0.502    0.000 locale.py:647(getpreferredencoding)
    27157    0.237    0.000    4.258    0.000 regex.py:328(findall)
    27157    0.346    0.000    0.929    0.000 regex.py:434(_compile)
  4815114    8.703    0.000   16.973    0.000 tokenization_gpt2.py:139(bpe)
  2030532    1.171    0.000    1.923    0.000 tokenization_gpt2.py:149(<lambda>)
    27157   22.988    0.001  112.449    0.004 tokenization_gpt2.py:180(_tokenize)
 25459527    5.708    0.000    5.708    0.000 tokenization_gpt2.py:194(<genexpr>)
 10242602    1.755    0.000    1.755    0.000 tokenization_gpt2.py:195(<genexpr>)
  1377876    1.595    0.000    1.900    0.000 tokenization_gpt2.py:91(get_pairs)
    95205    0.345    0.000    0.565    0.000 tokenization_utils.py:1043(special_tokens_map)
    95205    0.581    0.000    1.236    0.000 tokenization_utils.py:1055(all_special_tokens)
        1    0.022    0.022  114.720  114.720 tokenization_utils.py:615(tokenize)
    40789    0.103    0.000    0.182    0.000 tokenization_utils.py:623(split_on_token)
        1    0.583    0.583  114.698  114.698 tokenization_utils.py:641(split_on_tokens)
    54417    0.248    0.000  113.314    0.002 tokenization_utils.py:659(<genexpr>)
    27157    0.032    0.000    0.032    0.000 {built-in method _locale.nl_langinfo}
    81471    0.111    0.000    0.111    0.000 {built-in method _locale.setlocale}
   761640    0.219    0.000    0.219    0.000 {built-in method builtins.getattr}
    54314    0.012    0.000    0.012    0.000 {built-in method builtins.hasattr}
   516605    0.097    0.000    0.097    0.000 {built-in method builtins.isinstance}
  1821447    0.166    0.000    0.166    0.000 {built-in method builtins.len}
   472563    3.855    0.000    5.777    0.000 {built-in method builtins.min}
        1    0.000    0.000    0.000    0.000 {built-in method from_iterable}
  2043214    0.305    0.000    0.305    0.000 {method 'add' of 'set' objects}
   456488    0.058    0.000    0.058    0.000 {method 'append' of 'list' objects}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
  4815114    1.104    0.000    1.104    0.000 {method 'encode' of 'str' objects}
  5550977   17.434    0.000   19.189    0.000 {method 'extend' of 'list' objects}
    27157    3.092    0.000    3.092    0.000 {method 'findall' of '_regex.Pattern' objects}
  2057689    0.759    0.000    0.759    0.000 {method 'get' of 'dict' objects}
   735863    0.243    0.000    0.243    0.000 {method 'index' of 'tuple' objects}
  4894984   41.030    0.000   46.738    0.000 {method 'join' of 'str' objects}
        1    0.000    0.000    0.000    0.000 {method 'keys' of 'dict' objects}
  4855903    1.396    0.000    1.396    0.000 {method 'split' of 'str' objects}
    68048    0.009    0.000    0.009    0.000 {method 'strip' of 'str' objects}
    95205    0.013    0.000    0.013    0.000 {method 'values' of 'dict' objects}
```

It significantly improves the speed as seen in the difference between 114 seconds and 1566 seconds.

I'm going to create a pull request if everything is clear?
Thank you for your effort.
Thank you for taking the time to look into this and opening a pull request!<|||||>You are welcome,
This thing has big effort and I always support as a community member :) .
If you merge PR I would be appreciated because I want to use it originally as provided in the master branch, for my ongoing project.
Thanks!<|||||>Your PR was merged, you can now use it from the master branch :)
Feel free to open other issues if you find other sub-optimal processes.