Add CMU Hinglish DoG Dataset for MT
Hi @lhoestq, thanks a lot for the help. I have moved the part as suggested. 
Although still while running the dummy data script, I face this issue

```
Traceback (most recent call last):
  File "/home/ishan/anaconda3/bin/datasets-cli", line 8, in <module>
    sys.exit(main())
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/commands/datasets_cli.py", line 33, in main
    service.run()
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/commands/dummy_data.py", line 318, in run
    self._autogenerate_dummy_data(
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/commands/dummy_data.py", line 363, in _autogenerate_dummy_data
    dataset_builder._prepare_split(split_generator)
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/builder.py", line 1103, in _prepare_split
    example = self.info.features.encode_example(record)
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/features/features.py", line 981, in encode_example
    return encode_nested_example(self, example)
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/features/features.py", line 775, in encode_nested_example
    return {
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/features/features.py", line 775, in <dictcomp>
    return {
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 99, in zip_dict
    yield key, tuple(d[key] for d in dicts)
  File "/home/ishan/anaconda3/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 99, in <genexpr>
    yield key, tuple(d[key] for d in dicts)
KeyError: 'status'
```
This KeyError is at times different from 'status' also.
when I run 
```
datasets-cli dummy_data datasets/cmu_hinglish_dog --auto_generate --json_field='history'
```
I have tried removing unnecessary feature type definition, but that didn't help. Please let me know if I am missing something, thanks! The CI fail is unrelated to this PR and fixed on master. Merging !
Address part of #2841 

Added the CMU Hinglish DoG Dataset as in GLUECoS. Added it as a seperate dataset as unlike other tasks of GLUE CoS this can't be evaluated for a BERT like model. 
Consists of parallel dataset between Hinglish (Hindi-English) and English, can be used for Machine Translation between the two. 

The data processing part is inspired from the GLUECoS repo [here](https://github.com/microsoft/GLUECoS/blob/7fdc51653e37a32aee17505c47b7d1da364fa77e/Data/Preprocess_Scripts/preprocess_mt_en_hi.py)
The dummy data part is not working properly, it shows 
``` UnboundLocalError: local variable 'generator_splits' referenced before assignment ``` 
when I run without ``--auto_generate``.

Please let me know how I can fix that.
Thanks