Add E2E support for THD format

[xiaoyao0115]

Description

This PR adds Sequence Packing (THD format) E2E support to MCore. Dev branch PR:#2924

The core missing functionalities of THD in MCore are:

• data iterator cannot handle THD meta data, like cu_seqlens, max_seqlens.
• num_microbatches is fixed.
• PackParams are not passing between PP ranks.

Key Changes

1. Add a data_iterator wrapper (megatron/core/datasets/data_schedule.py::wrap_dataloader)

A wrapper function that intercepts the data iterator to perform scheduling and packing:

• Schedule & Pack: Extracts data from the data iterator, schedules sequences across DP×CP ranks, and packs them into microbatches with cu_seqlens metadata.
• Returns packing results: Returns the packed num_microbatches along with two parameters for FLOPs calculation: num_total_tokens_this_global_batch and sequence_square_sum_this_global_batch.
• TP broadcast: Broadcasts num_microbatches and FLOPs parameters across TP ranks since only TP rank 0 has access to the data iterator.
• PP broadcast: When using PP, middle PP stages (not first or last) require metadata (cu_seqlens, cu_seqlens_padded, max_seqlen, etc.) to be broadcast from PP rank 0 for correct computation.

2. Mock SFT Dataset Support

Supports mock datasets for testing and benchmarking with configurable sequence length distributions.
There are two modes of mock sft dataset:

• File mode: Load sequence lengths from an external CSV, example json:

  {"mode": "file", "path": "/path/to/seqlens.csv"}

• Distribution mode: Generate sequence lengths from a distribution (currently supports lognormal), example json:

  {"mode": "distribution", "type": "lognormal", "min_seq_len": 1024, "max_seq_len": 8192, "mean_seq_len": 4096, "lognormal_sigma": 1.1}

Architecture

Before vs After

graph LR
    subgraph Before
        A1[DataIterator] --> B1[get_batch]
        B1 --> C1[forward_backward]
        C1 --> D1[Fixed seq_len FLOPs]
    end
    subgraph After
        A2[DataIterator] --> W[wrap_dataloader]
        W -->|schedule + pack| B2[PackedDataIterator]
        W -->|broadcast| M[num_microbatches + flops_params]
        B2 --> C2[get_batch_for_sequence_packing]
        C2 --> D2[forward_backward]
        D2 --> E2[Dynamic FLOPs]
        M   --> E2
    end

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Execution Flow

sequenceDiagram
    participant Train as training.py
    participant Schedule as schedules.py
    participant Wrap as wrap_iterator_helper
    participant DataSched as data_schedule.py
    participant GetBatch as get_batch_for_seq_packing

    Train->>Schedule: forward_backward_*(data_iterator)
    Schedule->>Wrap: wrap_iterator_helper(config, data_iterator)
    Wrap->>DataSched: wrap_dataloader(data_iterator, scheduler_type)
    
    Note over DataSched: 1. Gather global seqlens across DP
    Note over DataSched: 2. Scheduler assigns sequences to microbatches
    Note over DataSched: 3. All-to-all redistribute samples
    Note over DataSched: 4. Pack into microbatches
    Note over DataSched: 5. Broadcast to TP/PP ranks
    
    DataSched-->>Schedule: (packed_iter, num_mbs, total_tokens, seq_sq_sum)
    
    loop for each microbatch
        Schedule->>GetBatch: get_batch_on_this_rank_for_sequence_packing
        Note over GetBatch: Broadcast tokens/labels to TP group
        Note over GetBatch: Partition for CP if needed
        GetBatch-->>Schedule: (tokens, labels, loss_mask, pos_ids, packed_seq_params)
    end
    
    Schedule-->>Train: forward_data_store + [total_tokens, seq_sq_sum]

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New Arguments

Argument	Type	Description
--sequence-packing	flag	Enable sequence packing (THD format) for training
--sequence-packing-scheduler	str	Scheduler type: default or empty
--sft-mock-dataset-config-json	str	JSON config for mock dataset

Changes

File	Description
megatron/core/datasets/data_schedule.py	Core scheduling and packing logic
megatron/core/pipeline_parallel/schedules.py	Integration with forward/backward schedules
megatron/training/training.py	Updated FLOPs calculation for variable-length sequences
megatron/training/datasets/sft_dataset.py	Mock dataset support
megatron/training/arguments.py	New CLI arguments
megatron/core/model_parallel_config.py	Configuration options
tests/unit_tests/test_sequence_packing.py	Unit tests

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