As I'm a Full-stack developer at Core. I'd be looking to extend the TRAINS Frontend and Backend APIs to suit my need of On-Prem data storage integration and lots of other customization for Job Scheduler(CRON)/Dataset Augmentation/Custom Annot. tool etc.

That is awesome! Feel free to post a specific question here, and I'll try to direct to the right place 🙂

Can you guide me to one such tutorial that's teaching how to customize the backend/front end with an example?

You mean like pipelines / automation etc.?
I f that is the case take a look at the examples folder :
https://github.com/allegroai/trains/tree/master/examples
Mostly the automation and services subfolders.
And also on the trains-agent examples (with the AWS autoscaler example, that soon will be rewritten so it is easier to extend )
https://github.com/allegroai/trains-agent/tree/master/examples