Extract args init to launch scripts

[maanug-nv]

What does this PR do ?

Extract args parsing and validation into a dedicated function out of initialize_megatron(), and call it in pretrain scripts. This is to facilitate separating args logic from training code.

We will build the config classes from the args in the pretrain scripts and pass that to pretrain(), in #4227 .

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[maanug-nv]

/claude review

[claude]

Two issues found:

1. Bug: parse_and_validate_args missing return args — commented inline on megatron/training/arguments.py.

2. Dead parameter: parsed_args in initialize_megatron — the parsed_args=None parameter on initialize_megatron() is now unused. The function body no longer references it (it just calls get_args()). It should be removed to avoid confusion, along with any callers that pass it.

[maanug-nv]

Code review

Found 2 issues:

1. pretrain() no longer accepts extra_args_provider or args_defaults, but pretrain_bert.py, pretrain_t5.py, pretrain_vlm.py, and train_rl.py still pass these — they will raise TypeError at runtime.

Megatron-LM/megatron/training/training.py

Lines 817 to 830 in 14fedae

	def pretrain(
	train_valid_test_dataset_provider,
	model_provider,
	model_type,
	forward_step_func,
	process_non_loss_data_func=None,
	get_embedding_ranks=None,
	get_position_embedding_ranks=None,
	non_loss_data_func=None,
	store=None,
	inprocess_call_wrapper: Optional[CallWrapper] = None,
	):
	"""Main training program.

https://github.com/NVIDIA/Megatron-LM/blob/14fedaebc2dfc58d87021427ba52b82fdd24d4aa/pretrain_bert.py#L193-L197

2. In pretrain_mamba.py, from megatron.training.arguments import parse_and_validate_args was inserted before _PROGRAM_START_TIME = time.time(), violating the comment's stated intent to capture program start time before heavy imports. In pretrain_gpt.py the same import is correctly placed after the timestamp.

Megatron-LM/pretrain_mamba.py

Lines 5 to 9 in 14fedae

	import time
	from megatron.training.arguments import parse_and_validate_args
	_PROGRAM_START_TIME = time.time()

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[svcnvidia-nemo-ci]

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You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24422302488

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24427514360

[JordiBayarri]

Missing a update in "examples/multimodal/train.py"