"pymarian" CLI, a proxy to "marian"

[thammegowda]

Description

Added pymarian CLI which is proxy to the currently used marian binary. This enables PyPI based distribution of marian: pip install pymarian to get started, except replace marian -> pymarian in CLI.

List of changes:

• the main() function is now bound to pymarian.main to achieve this
• main() function now lists the subcommands if no arguments are given or explicitly "?" is passed. I am hoping this is useful for new users. Note: marian --help behavior is currently mapped to marian train --help, hence picked ? to avoid breaking changes
• added pymarian.help() method to list config options (printed to STDOUT). Maybe useful for new users in notebooks
• Minor improvements to pymarian.Evaluator class. model and vocab args can now be strings (previously Path)

How to test

# build and install pymarian asusual
cmake -B build -DPYMARIAN=on -DCOMPILE_CUDA=off .
cmake --build build -j
pip install build/pymarian*.whl
# try these commands

pymarian 
pymarian train --help
pymarian decode --help
pymarian evaluate --help

Checklist

• I have tested the code manually
• I have run regression tests
• I have read and followed CONTRIBUTING.md
• I have updated CHANGELOG.md

[thammegowda]

Moved this PR to internal fork. Hence closing here.