Ipad pro M1 doesn't offer OS 16 as an update.

Correct. Unlike iOS16 for iPhone, released 12th September (iOS 16.0.1 subsequently released 16th September), iPadOS16 is not due for release until sometime in October. 

However, iPadOS 15.7 was also released 12th September - incorporating essential security updates.

About the security content of iOS 15.7 and iPadOS 15.7 - Apple Support

While iOS16 is available for iPhone, iPhone owners have the option to just install iOS15.7 at this time.

More information about the forthcoming iPadOS16 can be found here:

iPadOS 16 Preview - Apple

Per Apple.

iPadOS 16 coming sometime in October 2022.

Tap this link below.

iPadOS 16 Preview - Apple

iPad OS 16 has not been released. While iOS 16 for iPhones has been, Apple has stated that iPad OS 16 will release in October.