@Raina, Vikas Thanks for reaching out to Microsoft Q&A. I understand that you want a report of all the Azure VMs and their corresponding Boot diagnostic Storage Accounts.
To Get all virtual machines in your subscription - Get-AzVM should provide you with the same.
To get a list of VMs in all your subscriptions, you will need a script for the same. Here is an example of the same.
As per Boot diagnostics enabled VMs, I will look into the same and get back to you if there is any command/script for the same. Thank you!