How do get the result of the last eval epoch from the json file like any other eval epoch?

## 🚀 Feature
I hope that there is a good way to get the result of the last eval epoch from the json file like any other eval epoch.

## Motivation & Examples

It seems that the result of the last eval epoch will only be printed in the command line window and not written directly to the json file. This caused some problems for me, for example, when I want to plot eval results over the epoch, I have to make some changes to the json file to make it easier to manipulate. Is there a convenient solution?

The issue does not name a file, test, or entry point; start by locating the evaluation logging and JSON-writing path in Detectron2. Confirm how intermediate evaluation epochs are serialized, then ensure the final evaluation result is written in the same format so epoch-based plotting needs no manual JSON changes.

Written by the indexing model from the issue text.