In google drive i found some buttons and labels are found to be inconsistent in cursors.
Are there commonly accepted conventions as to default cursors and pointer cursors in web and desktop applications?
In desktop or web application the default cursor is always been a convention for a button, for example, in HTML the default cursor for the button object is the default setting: if you want a pointer, you have to specify it via CSS.
It's all about affordance: the button object has a strong affordance, the user knows that it's an interactive object and how it works, as you can read on the Microsoft's guidelines for developer:
"[...]command buttons already have a strong affordance, so they don't need a hand pointer. The hand pointer must mean "this target is a link" and nothing else."
Although it is a convention, it's actually common to use the hand pointer: StackExchange does it too.