[feat] Allow HF callback to initialize Aim run at init end

[dushyantbehl]

Current implementation of HF callback in Aim initializes the Run object at training begin time. This means if the users want to track any additional metrics they need to do so before training finishes while using HFTrainer object.

This small change which initializes the Run object on init_end makes the run object available to user of this callback via callback.experiment and allows them to track any extra metrics or parameters with the same run other than the standard metrics output by HF.

[dushyantbehl]

@alberttorosyan sure!

[dushyantbehl]

@alberttorosyan done with the changes, let me know if I need to change anything. 🥂