BLEURT: Match key names to correspond with filename
Thanks for the suggestion! I think the current checked-in `CHECKPOINT_URLS` is already not working. I believe anyone who tried using the new ckpts (`BLEURT-20-X`) can't unless this fix is in. The zip file from bleurt side unzips to directory name matching the filename (capitalized for new ones). For example without current changes I'd get the following error

```
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-5-f6832fe20f84> in <module>()
      1 predictions = ["hello there", "general kenobi"]
      2 references = ["hello there", "general kenobi"]
----> 3 bleurt = datasets.load_metric("bleurt", "bleurt-20")
      4 results = bleurt.compute(predictions=predictions, references=references)

4 frames
/usr/local/lib/python3.7/dist-packages/bleurt/checkpoint.py in read_bleurt_config(path)
     84   """Reads and checks config file from a BLEURT checkpoint."""
     85   assert tf.io.gfile.exists(path), \
---> 86       "Could not find BLEURT checkpoint {}".format(path)
     87   config_path = os.path.join(path, CONFIG_FILE)
     88   assert tf.io.gfile.exists(config_path), \

AssertionError: Could not find BLEURT checkpoint /root/.cache/huggingface/metrics/bleurt/bleurt-20/downloads/extracted/e34c60f1a05394ecda54e253a10413ca7b5d59f9a23f3cc73258c6b78ffa2f50/bleurt-20
```
inspecting specified path I see that directory name is `BLEURT-20` instead of `bleurt-20`. 
Other solution similar to your suggestion is meddle with `dl_manager.download_and_extract` to unzip to paths with lowering all the paths but I imagine this will affect other parts of the library.  Indeed, good catch ! Your solution that fixes `CHECKPOINT_URLS ` is simple and works well, thanks :)

Furthermore to avoid breaking changes though we could also keep the support for the lowercase one:
```python
        if self.config_name.lower() in CHECKPOINT_URLS:
            checkpoint_name = self.config_name.lower()
        elif self.config_name.upper() in CHECKPOINT_URLS:
            checkpoint_name = self.config_name.upper()
        else:
            raise KeyError(
                f"{self.config_name} model not found. You should supply the name of a model checkpoint for bleurt in {CHECKPOINT_URLS.keys()}"
            )
```
and then we can use `checkpoint_name` instead of `self.config_name` to download and instantiate the model:
```python
        model_path = dl_manager.download_and_extract(CHECKPOINT_URLS[checkpoint_name])
        self.scorer = score.BleurtScorer(os.path.join(model_path, checkpoint_name))
```

Please let me know if that sounds reasonable to you ! Thanks for the suggestion! I believe your suggestion should work to make keys case insensitive. Changes are committed to the PR now. 
In order to properly locate downloaded ckpt files key name needs to match filename. Correcting change introduced in #3235 