Rename import lib 'megatron' to 'megatron_util' and add error message for users when import failed.
Use initialize_megatron as a unified initialization entry in megatron-util, which can accept configuration input of ConfigDict in MaaS-lib.
Wrap the initialization process into the utils/megatron_utils.py file, add default parameters for the existing large model to be compatible with the uploaded configuration file.
The version of megatron_cfg currently supports v3 (default), v1 and moe.
Link: https://code.alibaba-inc.com/Ali-MaaS/MaaS-lib/codereview/11231840
