medical_dialog zh has very slow _generate_examples
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Hi @nbroad1881, thanks for reporting.

Let me have a look to try to improve its performance.  Thanks @nbroad1881  for reporting! I don't recall it taking so long. I will also have a look at this. 
@albertvillanova please let me know if I am doing something unnecessary or time consuming. Hi @nbroad1881 and @vrindaprabhu,

As a workaround for the performance of the parsing of the raw data files (this could be addressed in a subsequent PR), I have found that there are also processed data files, that do not require parsing. I have added these as new configurations `processed.en` and `processed.zh`:
```python
ds = load_dataset("medical_dialog", "processed.zh")
```
## Describe the bug
After downloading the files from Google Drive, `load_dataset("medical_dialog", "zh", data_dir="./")` takes an unreasonable amount of time. Generating the train/test split for 33% of the dataset takes over 4.5 hours.

## Steps to reproduce the bug
The easiest way I've found to download files from Google Drive is to use `gdown` and use Google Colab because the download speeds will be very high due to the fact that they are both in Google Cloud.

```python
file_ids = [
        "1AnKxGEuzjeQsDHHqL3NqI_aplq2hVL_E",
        "1tt7weAT1SZknzRFyLXOT2fizceUUVRXX",
        "1A64VBbsQ_z8wZ2LDox586JIyyO6mIwWc",
        "1AKntx-ECnrxjB07B6BlVZcFRS4YPTB-J",
        "1xUk8AAua_x27bHUr-vNoAuhEAjTxOvsu",
        "1ezKTfe7BgqVN5o-8Vdtr9iAF0IueCSjP",
        "1tA7bSOxR1RRNqZst8cShzhuNHnayUf7c",
        "1pA3bCFA5nZDhsQutqsJcH3d712giFb0S",
        "1pTLFMdN1A3ro-KYghk4w4sMz6aGaMOdU",
        "1dUSnG0nUPq9TEQyHd6ZWvaxO0OpxVjXD",
        "1UfCH05nuWiIPbDZxQzHHGAHyMh8dmPQH",
]
for i in file_ids:
    url = f"https://drive.google.com/uc?id={i}"
    !gdown $url


from datasets import load_dataset

ds = load_dataset("medical_dialog", "zh", data_dir="./")
```

## Expected results
Faster load time

## Actual results
`Generating train split: 33%: 625519/1921127 [4:31:03<31:39:20, 11.37 examples/s]`

## Environment info
- `datasets` version: 2.0.0
- Platform: Linux-5.4.144+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.13
- PyArrow version: 6.0.1
- Pandas version: 1.3.5

@vrindaprabhu , could you take a look at this since you implemented it? I think the `_generate_examples` function might need to be rewritten
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