Update callbacks.py
This pull request **introduces 1 alert** when merging 815f56384103352d86009929db2d563bcfd025be into 941ef1fd71bd2515a4ba7092d65146edfddc1229 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-0a5876cb39e3cfc1fc9406351d69ccb93bcbcd7a)

**new alerts:**

* 1 for Unused import