UserWarning: The given NumPy array is not writeable, and PyTorch does not support non-writeable tensors
@thomwolf @AndreasMadsen I think this is a terrible idea, n/o, and I am very much against it. Modifying internals of an array in such a hacky way is bound to run into other (user) issues down the line. To users it would not be clear at all what is going on e.g. when they check for write access (which will return True) but then they get a warning that the array is not writeable. That's extremely confusing. 

If your only goal is to get rid of warnings in your code, then you can just use a [simplefilter](https://docs.python.org/3.8/library/warnings.html#temporarily-suppressing-warnings) for UserWarnings in your own code. Changing the code-base in such an intuitive way does not seem like a good way to go and sets a bad precedent, imo. 

(Feel free to disagree, of course.)

IMO a warning can stay (as they can be filtered by users anyway), but it can be clarified why the warning takes place.
I am trying out the library and want to load in pickled data with `from_dict`. In that dict, one column `text` should be tokenized and the other (an embedding vector) should be retained. All other columns should be removed. When I eventually try to set the format for the columns with `set_format` I am getting this strange Userwarning without a stack trace:

> Set __getitem__(key) output type to torch for ['input_ids', 'sembedding'] columns  (when key is int or slice) and don't output other (un-formatted) columns.
> C:\Users\bramv\.virtualenvs\dutch-simplification-nbNdqK9u\lib\site-packages\datasets\arrow_dataset.py:835: UserWarning: The given NumPy array is not writeable, and PyTorch does not support non-writeable tensors. This means you can write to the underlying (supposedly non-writeable) NumPy array using the tensor. You may want to copy the array to protect its data or make it writeable before converting it to a tensor. This type of warning will be suppressed for the rest of this program. (Triggered internally at  ..\torch\csrc\utils\tensor_numpy.cpp:141.)
>   return torch.tensor(x, **format_kwargs)

The first one might not be related to the warning, but it is odd that it is shown, too. It is unclear whether that is something that I should do or something that that the program is doing at that moment.

Snippet:
```
    dataset = Dataset.from_dict(torch.load("data/dummy.pt.pt"))
    print(dataset)
    tokenizer = AutoTokenizer.from_pretrained("bert-base-cased")
    keys_to_retain = {"input_ids", "sembedding"}
    dataset = dataset.map(lambda example: tokenizer(example["text"], padding='max_length'), batched=True)
    dataset.remove_columns_(set(dataset.column_names) - keys_to_retain)

    dataset.set_format(type="torch", columns=["input_ids", "sembedding"])
    dataloader = torch.utils.data.DataLoader(dataset, batch_size=2)

    print(next(iter(dataloader)))
```

PS: the input type for `remove_columns_` should probably be an Iterable rather than just a List.
UserWarning: The given NumPy array is not writeable, and PyTorch does not support non-writeable tensors 
 I am trying out the library and want to load in pickled data with `from_dict`. In that dict, one column `text` should be tokenized and the other (an embedding vector) should be retained. All other columns should be removed. When I eventually try to set the format for the columns with `set_format` I am getting this strange Userwarning without a stack trace:

> Set __getitem__(key) output type to torch for ['input_ids', 'sembedding'] columns  (when key is int or slice) and don't output other (un-formatted) columns.
> C:\Users\bramv\.virtualenvs\dutch-simplification-nbNdqK9u\lib\site-packages\datasets\arrow_dataset.py:835: UserWarning: The given NumPy array is not writeable, and PyTorch does not support non-writeable tensors. This means you can write to the underlying (supposedly non-writeable) NumPy array using the tensor. You may want to copy the array to protect its data or make it writeable before converting it to a tensor. This type of warning will be suppressed for the rest of this program. (Triggered internally at  ..\torch\csrc\utils\tensor_numpy.cpp:141.)
>   return torch.tensor(x, **format_kwargs)

The first one might not be related to the warning, but it is odd that it is shown, too. It is unclear whether that is something that I should do or something that that the program is doing at that moment.

Snippet:
```
    dataset = Dataset.from_dict(torch.load("data/dummy.pt.pt"))
    print(dataset)
    tokenizer = AutoTokenizer.from_pretrained("bert-base-cased")
    keys_to_retain = {"input_ids", "sembedding"}
    dataset = dataset.map(lambda example: tokenizer(example["text"], padding='max_length'), batched=True)
    dataset.remove_columns_(set(dataset.column_names) - keys_to_retain)

    dataset.set_format(type="torch", columns=["input_ids", "sembedding"])
    dataloader = torch.utils.data.DataLoader(dataset, batch_size=2)

    print(next(iter(dataloader)))
```

PS: the input type for `remove_columns_` should probably be an Iterable rather than just a List. 
 @thomwolf @AndreasMadsen I think this is a terrible idea, n/o, and I am very much against it. Modifying internals of an array in such a hacky way is bound to run into other (user) issues down the line. To users it would not be clear at all what is going on e.g. when they check for write access (which will return True) but then they get a warning that the array is not writeable. That's extremely confusing. 

If your only goal is to get rid of warnings in your code, then you can just use a [simplefilter](https://docs.python.org/3.8/library/warnings.html#temporarily-suppressing-warnings) for UserWarnings in your own code. Changing the code-base in such an intuitive way does not seem like a good way to go and sets a bad precedent, imo. 

(Feel free to disagree, of course.)

IMO a warning can stay (as they can be filtered by users anyway), but it can be clarified why the warning takes place.