[Bug]: When use OpenAI chat model , raise ERROR: 'CompletionUsage'
object has no attribute 'get' #2948
### What problem does this PR solve?
the detail of this PR is shown at
https://github.com/infiniflow/ragflow/issues/2948
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>