How to choose proper download_mode in function load_dataset?
`download_mode=datasets.GenerateMode.FORCE_REDOWNLOAD` should work.
This makes me think we we should rename this to DownloadMode.FORCE_REDOWNLOAD. Currently that's confusing Can we just use `features=...` in `load_dataset` for this @lhoestq? Indeed you should use `features` in this case. 
```python
features = Features({'text': Value('string'), 'label': Value('float32')})
dataset = load_dataset('csv', data_files=['sst_test.csv'], features=features)
```
Note that because of an issue with the caching when you change the features (see #750 ) you still need to specify the `FORCE_REDOWNLOAD ` flag. I'm working on a fix for this one https://github.com/huggingface/datasets/issues/769#issuecomment-717837832
> This makes me think we we should rename this to DownloadMode.FORCE_REDOWNLOAD. Currently that's confusing

@lhoestq do you still think we should rename it?
 It's no big deal, but since it can be confusing to users I think it's worth renaming it, and deprecate `GenerateMode` until `datasets` 2.0 at least. IMO it's confusing to have `download_mode=GenerateMode.something`
Hi, I am a beginner to datasets and I try to use datasets to load my csv file.
my csv file looks like this

``` 
text,label
"Effective but too-tepid biopic",3
"If you sometimes like to go to the movies to have fun , Wasabi is a good place to start .",4
"Emerges as something rare , an issue movie that 's so honest and keenly observed that it does n't feel like one .",5
```

First I try to use this command to load my csv file .  

``` python
dataset=load_dataset('csv', data_files=['sst_test.csv'])
```

It seems good, but when i try to overwrite the convert_options to convert  'label' columns from int64 to float32 like this.

``` python
import pyarrow as pa
from pyarrow import csv
read_options = csv.ReadOptions(block_size=1024*1024)
parse_options = csv.ParseOptions()
convert_options = csv.ConvertOptions(column_types={'text': pa.string(), 'label': pa.float32()})
dataset = load_dataset('csv', data_files=['sst_test.csv'], read_options=read_options,
                       parse_options=parse_options, convert_options=convert_options)
```

It keeps the same:

```shell
Dataset(features: {'text': Value(dtype='string', id=None), 'label': Value(dtype='int64', id=None)}, num_rows: 2210)
```

I think this issue is caused by the parameter "download_mode" Default to REUSE_DATASET_IF_EXISTS because after I delete the cache_dir, it seems right.

Is it a bug? How to choose proper download_mode to avoid this issue?
