Error about load_metric
Hi ! I wasn't able to reproduce your error.

Can you try to clear your cache at `~/.cache/huggingface/modules` and try again ?
## Describe the bug
  File "/opt/conda/lib/python3.8/site-packages/datasets/load.py", line 1371, in load_metric
    metric = metric_cls(
TypeError: 'NoneType' object is not callable

## Steps to reproduce the bug
```python
metric = load_metric("glue", "sst2")
```


## Environment info
- `datasets` version: 1.16.1
- Platform: Linux-4.15.0-161-generic-x86_64-with-glibc2.10
- Python version: 3.8.3
- PyArrow version: 6.0.1
