read metric glue.py from local file 
### Describe the bug

Currently, The server is off-line.  I am using the glue metric from the local file downloaded from the hub. 
I download / cached datasets using `load_dataset('glue','sst2', cache_dir='/xxx')` to cache them and then in the off-line mode, I use `load_dataset('xxx/glue.py','sst2', cache_dir='/xxx')`. I can successfully reuse cached datasets.

My problem is about the load_metric. 
When I run `load_dataset('xxx/glue_metric.py','sst2',cache_dir='/xxx')` , it returns 

`  File "xx/lib64/python3.9/site-packages/datasets/utils/deprecation_utils.py", line 46, in wrapper
    return deprecated_function(*args, **kwargs)
  File "xx//lib64/python3.9/site-packages/datasets/load.py", line 1392, in load_metric
    metric = metric_cls(
TypeError: 'NoneType' object is not callable`

Thanks in advance for help! 
### Steps to reproduce the bug

N/A

### Expected behavior

N/A

### Environment info

`datasets == 2.12.0`