Adding multistage task type

[patrick-scale]

Pull Request Summary

Description

Please include a summary of the changes. Please also include the relevant context and motivation. List any dependencies and assumptions that are required for this change.

How did you test your code?

Which of the following have you done to test your changes? Please describe the tests that you ran to verify your changes.

• Created new unit tests in tests/ for the newly implemented methods
• Updated existing unit tests in tests/ to cover changes made to existing methods

Checklist

Please make sure all items in this checklist have been fulfilled before sending your PR out for review!

• I have commented my code in details, particularly in hard-to-understand areas
• I have updated Readme.rst document with examples for newly implemented public methods
• I have reviewed Deployment and Publishing Guide for Python SDK document
• I incremented the SDK version in _version.py (unless this PR only updates the documentation).
• In order to release a new version, a "Release Summary" needs to be prepared and published after the merge

[fatihkurtoglu]

Let's also bump the version file with a new sub version

[patrick-scale]

Let's also bump the version file with a new sub version

I bumped to version = "2.15.12", I could do "2.15.21" instead. let me know @fatihkurtoglu