Issue title: setting threshold for Speaker_change_detection 
 Issue body: tested code:
```
Speaker_change_detection = torch.hub.load('pyannote/pyannote-audio', 'scd_ami', pipeline=True)
Speaker_change_detection(OWN_FILE)
```
**Is your feature request related to a problem? Please describe.**
Is there a threshold that defines the change points?
I couldn't find a way to set a threshold  (if there is one) for the Speaker_change_detection.

**Describe the solution you'd like**
It would be useful to expose raw thresholds information and enable setting a threshold externally.

**other**
The [paper](https://github.com/yinruiqing/change_detection/blob/master/doc/change-detection.pdf) does specify that there is such a threshold. 

 Comments: 
 Comment 0: This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

{'source': 'GithubIssues', 'repo_name_with_owner': 'pyannote/pyannote-audio', 'issue_id': 1116564001, 'repo_stars': 1865.0}