Fix script of Arabic Billion Words dataset to return all data
The script has a bug and only parses and generates a portion of the entire dataset.

This PR fixes the loading script so that is properly parses the entire dataset. 

Current implementation generates the same number of examples as reported in the [original paper](https://arxiv.org/abs/1611.04033) for all configurations except for one:
- For "Youm7" we generate more examples (1172136) than the ones reported by the paper (1025027)

|                | Number of examples | Number of examples according to the source |
|:---------------|-------------------:|-----:|
| Alittihad      |             349342 |349342 |
| Almasryalyoum  |             291723 |291723 |
| Almustaqbal    |             446873 |446873 |
| Alqabas        |             817274 |817274 |
| Echoroukonline |             139732 |139732 |
| Ryiadh         |             858188 | 858188 |
| Sabanews       |              92149 |92149 |
| SaudiYoum      |             888068 |888068 |
| Techreen       |             314597 |314597 |
| Youm7          |            1172136 |1025027 |

Fix #3126.