The following is answer I got from azure support team on this -
"Unfortunately, Autoscaling feature on Virtual machine availabilty sets is natively support by Azure kubernetes for now. We have VMSS autoscaler feature and that too in preview phase."
They were focusing on manual scaling for now.
They also mentioned one github repo to refer but azure won't provide any support for it.
Its mentioned as follows -
I have done a quick research ,please find the github link where we have procedure on autoscaling of VM availbility sets, Kindly go through standard deployment section in the link. This is not directly supported by us and if you have any issues or concerns you can approach github for the same.
click here