Add MetaShift dataset
This PR adds the MetaShift dataset.

Dataset Request  : Add MetaShift dataset [#3813](https://github.com/huggingface/datasets/issues/3813)

@lhoestq As discussed, 
- I have copied the preprocessing script and modified it as required to not create new directories and folders and instead yield the images. 
-  I do the preprocessing in _split_generators to get the required data which is then passed to _generate_examples.
-  Beyond the generated MetaShift dataset, the original preprocess script also generates the meta-graphs for each class, I have currently not included this part. [ Ref : [Link](https://github.com/Weixin-Liang/MetaShift#generate-full-metashift) ]
-  There is a Bonus section, the authors share.  I have currently not included this part. [ Ref : [Link](https://github.com/Weixin-Liang/MetaShift#bonus-generate-the-metashift-attributes-dataset-subsets-defined-by-subject-attributes) ]
- I had a basic test script which downloaded the dataset and tested the basic functionality. Things seems fine.
For real data, I performed the following test : 
```
RUN_SLOW=1 pytest tests/test_dataset_common.py::LocalDatasetTest::test_load_real_dataset_metashift
============================================== test session starts ===============================================
platform linux -- Python 3.7.11, pytest-7.0.1, pluggy-1.0.0
rootdir: ./datasets
plugins: hydra-core-1.1.1, datadir-1.3.1, forked-1.4.0, xdist-2.5.0
collected 1 item                                                                                                 

tests/test_dataset_common.py .                                                                             [100%]

========================================= 1 passed in 4821.25s (1:20:21) =========================================
```
- I couldn't get the dummy dataset. Need some inputs here.
Error as follows : 
```
Using custom data configuration default
Dataset metashift with config None seems to already open files in the method `_split_generators(...)`. You might consider to instead only open files in the method `_generate_examples(...)` instead. If this is not possible the dummy data has to be created with less guidance. Make sure you create the file dummy_data/full-candidate-subsets.pkl.
    for split in generator_splits:
UnboundLocalError: local variable 'generator_splits' referenced before assignment
```

To-Do : 

- [x] Currently I am using the default _SELECTED_CLASSES. I need to use config option here as suggested
- [x] Complete fields in the Dataset Card.
- [x] Tagging the dataset using the Datasets Tagging app. 

Need your help and suggestions for improvement. Thank you
