groupby methods can return Series or DataFrame based on `as_index` variable

[loicdiridollou]

Describe the bug
Depending on the boolean as_index, methods on groupby objects will return different type.
This is a follow up for #203,

To Reproduce

1. Provide a minimal runnable pandas example that is not properly checked by the stubs.
2. Indicate which type checker you are using (mypy or pyright).
3. Show the error message received from that type checker while checking your example.

Please complete the following information:

• OS: [e.g. Windows, Linux, MacOS]
• OS Version [e.g. 22]
• python version
• version of type checker
• version of installed pandas-stubs

Additional context
Add any other context about the problem here.

[Dr-Irv]

Do you think you can create an inventory of methods here in this issue as a checklist, and then as you incrementally fix them, we can know which ones are fixed?

[loicdiridollou]

Yes let me add that here so we can see where we are at!

[loicdiridollou]

Outstanding methods to typehint:
✅ size
🔴 value_counts
Maybe there are other methods, feel free to append.

[Dr-Irv]

I tried a bunch of methods, and I think the only one left to worry about is value_counts().

[loicdiridollou]

Agreed, will take care of value_counts in a bit, I tried the other methods and they all return DataFrame whether we pass as_index as True or False.