load_dataset for CSV files not working
Hi, seems I also can't read csv file. I was trying with a dummy csv with only three rows.

```
text,label
I hate google,negative
I love Microsoft,positive
I don't like you,negative
```
I was using the HuggingFace image in Paperspace Gradient (datasets==1.1.3). The following code doesn't work:

```
from datasets import load_dataset
dataset = load_dataset('csv', script_version="master", data_files=['test_data.csv'], delimiter=",")
```
It outputs the following:
```
Using custom data configuration default
Downloading and preparing dataset csv/default-3b6254ff4dd403e5 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /root/.cache/huggingface/datasets/csv/default-3b6254ff4dd403e5/0.0.0/2960f95a26e85d40ca41a230ac88787f715ee3003edaacb8b1f0891e9f04dda2...
Dataset csv downloaded and prepared to /root/.cache/huggingface/datasets/csv/default-3b6254ff4dd403e5/0.0.0/2960f95a26e85d40ca41a230ac88787f715ee3003edaacb8b1f0891e9f04dda2. Subsequent calls will reuse this data.
```
But `len(dataset)` gives `1` and I can't access rows with indexing `dataset[0]` (it gives `KeyError: 0`).

However, loading from pandas dataframe is working.
```
from datasets import Dataset
import pandas as pd
df = pd.read_csv('test_data.csv')
dataset = Dataset.from_pandas(df)
```


Similar to #622, I've noticed there is a problem when trying to load a CSV file with datasets.

`
from datasets import load_dataset
`
`
dataset = load_dataset("csv", data_files=["./sample_data.csv"], delimiter="\t", column_names=["title", "text"], script_version="master")
`

Displayed error:
`
...
ArrowInvalid: CSV parse error: Expected 2 columns, got 1
`

I should mention that when I've tried to read data from `https://github.com/lhoestq/transformers/tree/custom-dataset-in-rag-retriever/examples/rag/test_data/my_knowledge_dataset.csv` it worked without a problem. I've read that there might be some problems with /r character, so I've removed them from the custom dataset, but the problem still remains.

I've added a colab reproducing the bug, but unfortunately I cannot provide the dataset.
https://colab.research.google.com/drive/1Qzu7sC-frZVeniiWOwzoCe_UHZsrlxu8?usp=sharing

Are there any work around for it ?
Thank you
load_dataset for CSV files not working 
 Similar to #622, I've noticed there is a problem when trying to load a CSV file with datasets.

`
from datasets import load_dataset
`
`
dataset = load_dataset("csv", data_files=["./sample_data.csv"], delimiter="\t", column_names=["title", "text"], script_version="master")
`

Displayed error:
`
...
ArrowInvalid: CSV parse error: Expected 2 columns, got 1
`

I should mention that when I've tried to read data from `https://github.com/lhoestq/transformers/tree/custom-dataset-in-rag-retriever/examples/rag/test_data/my_knowledge_dataset.csv` it worked without a problem. I've read that there might be some problems with /r character, so I've removed them from the custom dataset, but the problem still remains.

I've added a colab reproducing the bug, but unfortunately I cannot provide the dataset.
https://colab.research.google.com/drive/1Qzu7sC-frZVeniiWOwzoCe_UHZsrlxu8?usp=sharing

Are there any work around for it ?
Thank you 
 Hi, seems I also can't read csv file. I was trying with a dummy csv with only three rows.

```
text,label
I hate google,negative
I love Microsoft,positive
I don't like you,negative
```
I was using the HuggingFace image in Paperspace Gradient (datasets==1.1.3). The following code doesn't work:

```
from datasets import load_dataset
dataset = load_dataset('csv', script_version="master", data_files=['test_data.csv'], delimiter=",")
```
It outputs the following:
```
Using custom data configuration default
Downloading and preparing dataset csv/default-3b6254ff4dd403e5 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /root/.cache/huggingface/datasets/csv/default-3b6254ff4dd403e5/0.0.0/2960f95a26e85d40ca41a230ac88787f715ee3003edaacb8b1f0891e9f04dda2...
Dataset csv downloaded and prepared to /root/.cache/huggingface/datasets/csv/default-3b6254ff4dd403e5/0.0.0/2960f95a26e85d40ca41a230ac88787f715ee3003edaacb8b1f0891e9f04dda2. Subsequent calls will reuse this data.
```
But `len(dataset)` gives `1` and I can't access rows with indexing `dataset[0]` (it gives `KeyError: 0`).

However, loading from pandas dataframe is working.
```
from datasets import Dataset
import pandas as pd
df = pd.read_csv('test_data.csv')
dataset = Dataset.from_pandas(df)
```

