Issues in processing very large datasets
Hi ! `datasets` doesn't load the dataset in memory. Instead it uses memory mapping to load your dataset from your disk (it is stored as arrow files). Do you know at what point you have RAM issues exactly ?

How big are your graph_data_train dictionaries btw ?
## Describe the bug
I'm trying to add a feature called "subgraph" to CNN/DM dataset (modifications on run_summarization.py of Huggingface Transformers script) --- I'm not quite sure if I'm doing it the right way, though--- but the main problem appears when the training starts where the error ` [OSError: [Errno 12] Cannot allocate memory]`  appears. I suppose this problem roots in RAM issues and how the dataset is loaded during training, but I have no clue of what I can do to fix it.  Observing the dataset's cache directory, I see that it takes ~600GB of memory and that's why I believe special care is needed when loading it into the memory. 


Here are my modifications to `run_summarization.py` code. 


```
# loading pre-computed dictionary where keys are 'id' of article and values are corresponding subgraph
graph_data_train = get_graph_data('train') 
graph_data_validation = get_graph_data('val')
...
...


with training_args.main_process_first(desc="train dataset map pre-processing"):
    train_dataset = train_dataset.map(
        preprocess_function_train,
        batched=True,
        num_proc=data_args.preprocessing_num_workers,
        remove_columns=column_names,
        load_from_cache_file=not data_args.overwrite_cache,
        desc="Running tokenizer on train dataset",
    )

```


And here is the modified preprocessed function:

```
def preprocess_function_train(examples):
        inputs, targets, sub_graphs, ids = [], [], [], []
        for i in range(len(examples[text_column])):
            if examples[text_column][i] is not None and examples[summary_column][i] is not None:
                # if examples['doc_id'][i] in graph_data.keys():
                inputs.append(examples[text_column][i])
                targets.append(examples[summary_column][i])
                sub_graphs.append(graph_data_train[examples['id'][i]])
                ids.append(examples['id'][i])

        inputs = [prefix + inp for inp in inputs]
        model_inputs = tokenizer(inputs, max_length=data_args.max_source_length, padding=padding, truncation=True,
                                 sub_graphs=sub_graphs, ids=ids)

            # Setup the tokenizer for targets
        with tokenizer.as_target_tokenizer():
            labels = tokenizer(targets, max_length=max_target_length, padding=padding, truncation=True)

        # If we are padding here, replace all tokenizer.pad_token_id in the labels by -100 when we want to ignore
        # padding in the loss.
        if padding == "max_length" and data_args.ignore_pad_token_for_loss:
            labels["input_ids"] = [
                [(l if l != tokenizer.pad_token_id else -100) for l in label] for label in labels["input_ids"]
            ]

        model_inputs["labels"] = labels["input_ids"]
        return model_inputs
```


## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version:  2.1.0
- Platform: Linux Ubuntu
- Python version: 3.6
- PyArrow version: 6.0.1
