Pipeline parallelism

[le1nux]

What does this PR do?

This PR adds pipeline parallelism with generic interfaces for arbitrary model and a gpt2 model implementation

General Changes

• ..

Breaking Changes

• ..

Checklist before submitting final PR

• My PR is minimal and addresses one issue in isolation
• I have merged the latest version of the target branch into this feature branch
• I have reviewed my own code w.r.t. correct implementation, missing type hints, proper documentation, etc.
• I have run a sample config for model training
• I have checked that all tests run through (python tests/tests.py)
• I have updated the internal changelog (CHANGELOG_DEV.md)