add Toronto BooksCorpus

1. I knew there is a branch `toronto_books_corpus`
 - After I downloaded it, I found it is all non-english, and only have one row. 
- It seems that it cites the wrong paper
- according to papar using it, it is called `BooksCorpus` but not `TornotoBooksCorpus`

2. It use a text mirror in google drive
- `bookscorpus.py` include a function `download_file_from_google_drive` , maybe you will want to put it elsewhere.
- text mirror is found in this [comment on the issue](https://github.com/soskek/bookcorpus/issues/24#issuecomment-556024973), and it said to have the same statistics as the one in the paper.
- You may want to download it and put it on your gs in case of it disappears someday.

3. Copyright ?
The paper has said

> **The BookCorpus Dataset.** In order to train our sentence similarity model we collected a corpus of 11,038 books ***from the web***. These are __**free books written by yet unpublished authors**__. We only included books that had more than 20K words in order to filter out perhaps noisier shorter stories. The dataset has books in 16 different genres, e.g., Romance (2,865 books), Fantasy (1,479), Science fiction (786), Teen (430), etc. Table 2 highlights the summary statistics of our book corpus.

and we have changed the form (not books), so I don't think it should have that problems. Or we can state that use it at your own risk or only for academic use. I know @thomwolf should know these things more.

This should solved #131 

Comments:
Thanks for adding this one !

About the three points you mentioned:
1. I think the `toronto_books_corpus` branch can be removed @mariamabarham ? 
2. You can use the download manager to download from google drive. For you case you can just do something like 
```python
URL = "https://drive.google.com/uc?export=download&id=16KCjV9z_FHm8LgZw05RSuk4EsAWPOP_z"
...
arch_path = dl_manager.download_and_extract(URL)
```
Also this is is an unofficial host of the dataset, we should probably host it ourselves if we can.
3. Not sure about the copyright here, but I maybe @thomwolf has better insights about it. 

Yes it can be removed

I just downloaded the file and put it on gs. The public url is
https://storage.googleapis.com/huggingface-nlp/datasets/toronto_books_corpus/bookcorpus.tar.bz2

Could you try to change the url to this one and heck that everything is ok ?

In `books.py`
```
URL = "https://storage.googleapis.com/huggingface-nlp/datasets/toronto_books_corpus/bookcorpus.tar.bz2"
```
```
Python 3.7.6 (default, Jan  8 2020, 19:59:22) 
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from nlp import load_dataset
>>> book = load_dataset("nlp/datasets/bookscorpus/books.py", cache_dir='~/tmp')
Downloading and preparing dataset bookscorpus/plain_text (download: 1.10 GiB, generated: 4.52 GiB, total: 5.62 GiB) to /home/yisiang/tmp/bookscorpus/plain_text/1.0.0...
Downloading: 100%|███████████████████████████████████████████████████████████| 1.18G/1.18G [00:39<00:00, 30.0MB/s]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/yisiang/nlp/src/nlp/load.py", line 520, in load_dataset
    save_infos=save_infos,
  File "/home/yisiang/nlp/src/nlp/builder.py", line 420, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/home/yisiang/nlp/src/nlp/builder.py", line 460, in _download_and_prepare
    verify_checksums(self.info.download_checksums, dl_manager.get_recorded_sizes_checksums())
  File "/home/yisiang/nlp/src/nlp/utils/info_utils.py", line 31, in verify_checksums
    raise ExpectedMoreDownloadedFiles(str(set(expected_checksums) - set(recorded_checksums)))
nlp.utils.info_utils.ExpectedMoreDownloadedFiles: {'16KCjV9z_FHm8LgZw05RSuk4EsAWPOP_z'}
>>>
```

BTW, I notice the path `huggingface-nlp/datasets/toronto_books_corpus`, does it mean I have to change  folder name "bookscorpus" to "toronto_books_corpus"

> In `books.py`
> 
> ```
> URL = "https://storage.googleapis.com/huggingface-nlp/datasets/toronto_books_corpus/bookcorpus.tar.bz2"
> ```
> 
> ```
> Python 3.7.6 (default, Jan  8 2020, 19:59:22) 
> [GCC 7.3.0] :: Anaconda, Inc. on linux
> Type "help", "copyright", "credits" or "license" for more information.
> >>> from nlp import load_dataset
> >>> book = load_dataset("nlp/datasets/bookscorpus/books.py", cache_dir='~/tmp')
> Downloading and preparing dataset bookscorpus/plain_text (download: 1.10 GiB, generated: 4.52 GiB, total: 5.62 GiB) to /home/yisiang/tmp/bookscorpus/plain_text/1.0.0...
> Downloading: 100%|███████████████████████████████████████████████████████████| 1.18G/1.18G [00:39<00:00, 30.0MB/s]
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "/home/yisiang/nlp/src/nlp/load.py", line 520, in load_dataset
>     save_infos=save_infos,
>   File "/home/yisiang/nlp/src/nlp/builder.py", line 420, in download_and_prepare
>     dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
>   File "/home/yisiang/nlp/src/nlp/builder.py", line 460, in _download_and_prepare
>     verify_checksums(self.info.download_checksums, dl_manager.get_recorded_sizes_checksums())
>   File "/home/yisiang/nlp/src/nlp/utils/info_utils.py", line 31, in verify_checksums
>     raise ExpectedMoreDownloadedFiles(str(set(expected_checksums) - set(recorded_checksums)))
> nlp.utils.info_utils.ExpectedMoreDownloadedFiles: {'16KCjV9z_FHm8LgZw05RSuk4EsAWPOP_z'}
> >>>
> ```
> 
> BTW, I notice the path `huggingface-nlp/datasets/toronto_books_corpus`, does it mean I have to change folder name "bookscorpus" to "toronto_books_corpus"

Let me change the url to match "bookscorpus", so that you don't have to change anything. Good catch.

About the error you're getting: you just have to remove the `dataset_infos.json` and regenerate it

The new url is https://storage.googleapis.com/huggingface-nlp/datasets/bookscorpus/bookcorpus.tar.bz2

Hi, I found I made a mistake. I found the ELECTRA paper refer it as "BooksCorpus", but actually it is caleld "BookCorpus", according to the original paper. Sorry, I should have checked the original paper .

Can you do me a favor and change the url path to ` https://storage.googleapis.com/huggingface-nlp/datasets/bookcorpus/bookcorpus.tar.bz2` ?

Yep I'm doing it right now. Could you please rename all the references to `bookscorpus` and `BooksCorpus` to `book_corpus` and `BookCorpus` (with the right casing) ?

Thank you @lhoestq ,
Just to confirm it fits your naming convention
* make the file path `book_corpus/book_corpus.py` ?
* make `class Bookscorpus(nlp.GeneratorBasedBuilder)` -> `BookCorpus`  (which make cache folder name `book_corpus` and user use `load_dataset('book_corpus')`) ?
(Cuz I found "HellaSwag" dataset is named "nlp/datasets/hellaswag" and `class Hellaswag` )

Oh yea you're right about the Hellaswag example. We should keep the "_" symbol to replace spaces. As there are no space in BookCorpus, what we should do here is use:
- class name: 'Bookcorpus'
- script name: `bookcorpus/bookcorpus.py`
- use url https://storage.googleapis.com/huggingface-nlp/datasets/bookcorpus/bookcorpus.tar.bz2
And therefore the dataset name will be `bookcorpus`

Don't forget to regenerate the `dataset_infos.json` and we'll be good :D 

Awesome thanks :)