GitLab

* Fix megatron_gpt2 attention block's causal mask.

* compatibility with checkpoints created with recent versions of Megatron-LM

* added integration test for the released Megatron-GPT2 model

* code style changes

* added option to megatron conversion script to read from config file
Co-authored-by: Guido Novati <gnovati@nvidia.com>