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. 
 > 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.

Confusion can be resolved with a helpful error message. In this case, that error message can be controlled by huggingface/datasets. The right argument here is that if code depends on `.flags.writable` being truthful (not just for warnings), then it will cause unavoidable errors. Although, I can't imagine such a use-case.

> If your only goal is to get rid of warnings in your code, then you can just use a simplefilter 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.

I don't want to ignore all `UserWarnings`, nor all warnings regarding non-writable arrays. Ignoring warnings leads to hard to debug issues.

> IMO a warning can stay (as they can be filtered by users anyway), but it can be clarified why the warning takes place.

Plain use cases should really not generate warnings. It teaches developers to ignore warnings which is a terrible practice.

---

The best solution would be to allow non-writable arrays in `DataLoader`, but that is a PyTorch issue.