The process cannot access the file because it is being used by another process (windows)
I get the same behavior, on Windows, when `map`ping a function to a loaded dataset. 
The error doesn't occur if I re-run the cell a second time though! 
I'm on version 1.0.1.
Hi, I consistently get the following error when developing in my PC (windows 10):

```
    train_dataset = train_dataset.map(convert_to_features, batched=True)
  File "C:\Users\saareliad\AppData\Local\Continuum\miniconda3\envs\py38\lib\site-packages\nlp\arrow_dataset.py", line 970, in map
    shutil.move(tmp_file.name, cache_file_name)
  File "C:\Users\saareliad\AppData\Local\Continuum\miniconda3\envs\py38\lib\shutil.py", line 803, in move
    os.unlink(src)
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\Users\\saareliad\\.cache\\huggingface\\datasets\\squad\\plain_text\\1.0.0\\408a8fa46a1e2805445b793f1022e743428ca739a34809fce872f0c7f17b44ab\\tmpsau1bep1'

```
The process cannot access the file because it is being used by another process (windows) 
 Hi, I consistently get the following error when developing in my PC (windows 10):

```
    train_dataset = train_dataset.map(convert_to_features, batched=True)
  File "C:\Users\saareliad\AppData\Local\Continuum\miniconda3\envs\py38\lib\site-packages\nlp\arrow_dataset.py", line 970, in map
    shutil.move(tmp_file.name, cache_file_name)
  File "C:\Users\saareliad\AppData\Local\Continuum\miniconda3\envs\py38\lib\shutil.py", line 803, in move
    os.unlink(src)
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\Users\\saareliad\\.cache\\huggingface\\datasets\\squad\\plain_text\\1.0.0\\408a8fa46a1e2805445b793f1022e743428ca739a34809fce872f0c7f17b44ab\\tmpsau1bep1'

``` 
 I get the same behavior, on Windows, when `map`ping a function to a loaded dataset. 
The error doesn't occur if I re-run the cell a second time though! 
I'm on version 1.0.1.