[firewalled env] OFFLINE mode
Ok understand better the hanging issue.
I guess catching connection errors is not enough, we should also avoid all the hangings.
Currently the offline mode tests are only done by simulating an instant connection fail that returns an error, let's have another connection mock that hangs instead.

I'll also take a look at why you had to do this for `rouge`.

This issue comes from a need to be able to run `datasets` in a firewalled env, which currently makes the software hang until it times out, as it's unable to complete the network calls.

I propose the following approach to solving this problem, using the example of `run_seq2seq.py` as a sample program. There are 2 possible ways to going about it.

## 1. Manual

manually prepare data and metrics files, that is transfer to the firewalled instance the dataset and the metrics and run:

```
DATASETS_OFFLINE=1 run_seq2seq.py  --train_file xyz.csv --validation_file xyz.csv ...
```

`datasets` must not make any network calls and if there is a logic to do that and something is missing it should assert that this or that action requires network and therefore it can't proceed.

## 2. Automatic

In some clouds one can prepare a datastorage ahead of time with a normal networked environment but which doesn't have gpus and then one switches to the gpu instance which is firewalled, but it can access all the cached data. This is the ideal situation, since in this scenario we don't have to do anything manually, but simply run the same application twice:

1. on the non-firewalled instance:
```
run_seq2seq.py  --dataset_name wmt16 --dataset_config ro-en ...
```

which should download and cached everything.

2. and then immediately after on the firewalled instance, which shares the same filesystem
```
DATASETS_OFFLINE=1 run_seq2seq.py  --dataset_name wmt16 --dataset_config ro-en ...
```

and the metrics and datasets should be cached by the invocation number 1 and any network calls be skipped and if the logic is missing data it should assert and not try to fetch any data from online.

## Common Issues

1. for example currently `datasets` tries to look up online datasets if the files contain json or csv, despite the paths already provided

```
     if dataset and path in _PACKAGED_DATASETS_MODULES:
```

2. it has an issue with metrics. e.g. I had to manually copy `rouge/rouge.py` from the `datasets` repo to the current dir - or it was hanging.

I had to comment out `head_hf_s3(...)` calls to make things work. So all those `try: head_hf_s3(...)` shouldn't be tried with `DATASETS_OFFLINE=1`

Here is the corresponding issue for `transformers`: https://github.com/huggingface/transformers/issues/10379

Thanks.
[firewalled env] OFFLINE mode 
 This issue comes from a need to be able to run `datasets` in a firewalled env, which currently makes the software hang until it times out, as it's unable to complete the network calls.

I propose the following approach to solving this problem, using the example of `run_seq2seq.py` as a sample program. There are 2 possible ways to going about it.

## 1. Manual

manually prepare data and metrics files, that is transfer to the firewalled instance the dataset and the metrics and run:

```
DATASETS_OFFLINE=1 run_seq2seq.py  --train_file xyz.csv --validation_file xyz.csv ...
```

`datasets` must not make any network calls and if there is a logic to do that and something is missing it should assert that this or that action requires network and therefore it can't proceed.

## 2. Automatic

In some clouds one can prepare a datastorage ahead of time with a normal networked environment but which doesn't have gpus and then one switches to the gpu instance which is firewalled, but it can access all the cached data. This is the ideal situation, since in this scenario we don't have to do anything manually, but simply run the same application twice:

1. on the non-firewalled instance:
```
run_seq2seq.py  --dataset_name wmt16 --dataset_config ro-en ...
```

which should download and cached everything.

2. and then immediately after on the firewalled instance, which shares the same filesystem
```
DATASETS_OFFLINE=1 run_seq2seq.py  --dataset_name wmt16 --dataset_config ro-en ...
```

and the metrics and datasets should be cached by the invocation number 1 and any network calls be skipped and if the logic is missing data it should assert and not try to fetch any data from online.

## Common Issues

1. for example currently `datasets` tries to look up online datasets if the files contain json or csv, despite the paths already provided

```
     if dataset and path in _PACKAGED_DATASETS_MODULES:
```

2. it has an issue with metrics. e.g. I had to manually copy `rouge/rouge.py` from the `datasets` repo to the current dir - or it was hanging.

I had to comment out `head_hf_s3(...)` calls to make things work. So all those `try: head_hf_s3(...)` shouldn't be tried with `DATASETS_OFFLINE=1`

Here is the corresponding issue for `transformers`: https://github.com/huggingface/transformers/issues/10379

Thanks. 
 Ok understand better the hanging issue.
I guess catching connection errors is not enough, we should also avoid all the hangings.
Currently the offline mode tests are only done by simulating an instant connection fail that returns an error, let's have another connection mock that hangs instead.

I'll also take a look at why you had to do this for `rouge`.
