[Bugfix] to shared storage not working with dynamic PV provisioning

[NiccoloTosato]

This will fix #932

Summary

If shared storage is enabled, change HF_HOME from /data to /data/shared-pvc-storage

---

Checklist

• Make sure the code changes pass the pre-commit checks.
• Sign-off your commit by using -s when doing git commit
• Try to classify PRs for easy understanding of the type of changes, such as [Bugfix], [Feat], and [CI].

---

[gemini-code-assist]

Code Review

This pull request introduces support for shared PVC storage by updating the HF_HOME environment variable across vLLM deployments and Ray clusters. However, several critical issues were identified in the Helm templates. The logic in shared-storage.yaml incorrectly restricts PersistentVolume creation and storageClassName assignment when a storage class is defined, which breaks static provisioning. Additionally, while HF_HOME was updated in ray-cluster.yaml, the corresponding volume mounts for the shared storage are missing in both the head and worker groups, which will lead to runtime failures.