CodeInterpreter should not require a region argument

**Is your feature request related to a problem? Please describe.**
To create a `CodeInterpreter`, a `region` string argument must be passed. I use boto3 elsewhere in my application, and rely on its default region resolution mechanics. Now I need to explicit configure a region, which has to come top-down from my application.

**Describe the solution you'd like**
I should be able to create a `CodeInterpreter` without passing a region. It should default to using boto3's resolution for the region. If no region is configured, boto3 will raise an exception when performing an API operation.

**Describe alternatives you've considered**
I can manually create a session and get the region from it, then pass that same session to `CodeInterpreter`. This isn't quite right though, since `region_name` may be `None`. I could check if it's None and raise an exception myself, but having to do all this is awkward. Though I can also see the argument for "explicit is better than implicit."

```py
session = boto3.Session()
code_interpreter = CodeInterpreter(session.region_name, session)
```

Commencez par le constructeur de CodeInterpreter et examinez comment sa region et la session boto3 sont utilisées. Vérifiez le chemin de default-region décrit dans l’issue, puis confirmez que la création de CodeInterpreter sans region fonctionne et que boto3 ne signale toujours une erreur de région manquante que lorsqu’une opération d’API l’exige.

Rédigé par le modèle d'indexation à partir du texte de l'issue.