Replace whole-word `LossKwargs` with `TransformersKwargs` in modeling*.py

Initial upload from /leonardo_work/AIFAC_L01_028/hraj0000/megatron_lm_reference/converted_checkpoints/1.7b_data-MixtureVitae-300BT-instruct-80BT/hf/iter_0019377

initial commit