Fix validation error type `RT01` and check in CI

[MarckK]

Problem description

In order to have a continuous check by the CI on validation error RT01 (No Returns section found), fixing them in the code base enables the addition to the CI for automated testing in the future.

Please see overview of the errors.

Todo:

• get rid of the errors in the code base
• update the code_check.sh script to take into account the RT01 type of errors

[souvik3333]

I would like to work on this. But I have a doubt. What should I return so that the error gets fixed?

[gfyoung]

@souvik3333 : You should add a section describing what the function / method returns. Here are some examples that you can follow:

https://github.com/pandas-dev/pandas/blob/e3b095074c5c3b29ec25a994cd204349b872d961/pandas/core/dtypes/common.py

https://github.com/pandas-dev/pandas/blob/f4568fd76e864d8aee3d23f5a81302262d6e0dcb/pandas/io/excel/_util.py

[ihsansecer]

I could take this issue if nobody is working on it.