Datasets: Add WMT21 & WMT22 loaders (basic TSV loaders, sample data, tests)
closing since datasets should be added on https://huggingface.co directly
- Implemented TSV-based dataset loaders:
   - WMT21Dataset (local_datasets/wmt21/wmt21_dataset.py)
   - WMT22Dataset (local_datasets/wmt22/wmt22_dataset.py)
   These classes load source-target pairs from .tsv files for train, validation, and test splits.
- Created sample dummy data for both datasets:
   - dummy_data/train.tsv, dummy_data/validation.tsv, dummy_data/test.tsv
   - Includes a few realistic example lines to allow CI and local tests to pass without downloading full datasets.
- Added automated tests for robust validation:
   - tests/test_wmt21.py and tests/test_wmt22.py
   - Checks that all splits load correctly, empty lines are ignored, and the number of examples matches the number of lines in the .tsv files.
   - Edge cases handled: empty lines, malformed lines, extra tabs.
- Added README.md files for both datasets:
   - Provides dataset structure, usage instructions, and placeholders for citation & license information.
   - Ensures that other developers and reviewers can understand dataset usage immediately.
- Ensured easy local testing:
   - Load datasets programmatically using WMT21Dataset / WMT22Dataset.
   - Verified train/validation/test splits are correctly returned as Python dictionaries of Dataset objects.


-Provides initial support for WMT21 and WMT22 NLP/translation experiments.
-Allows contributors and reviewers to test dataset loading locally or in CI without downloading large datasets.
-Serves as a template to extend to other WMT datasets in the future.

Testing instructions:
```
# Activate your environment
pytest tests/test_wmt21.py -v
pytest tests/test_wmt22.py -v
```
```
from local_datasets.wmt21.wmt21_dataset import WMT21Dataset
from local_datasets.wmt22.wmt22_dataset import WMT22Dataset

# WMT21
wmt21 = WMT21Dataset("local_datasets/wmt21/dummy_data")
ds21 = wmt21.load()
print(ds21["train"][0])
print(ds21["validation"][0])
print(ds21["test"][0])

# WMT22
wmt22 = WMT22Dataset("local_datasets/wmt22/dummy_data")
ds22 = wmt22.load()
print(ds22["train"][0])
print(ds22["validation"][0])
print(ds22["test"][0])
```