feat: add DualPipe bidirectional pipeline schedule

[lishuangyuly]

FlagScale lacked support for the DualPipe bidirectional pipeline schedule introduced in DeepSeek-V3, which overlaps forward/backward computation across both pipeline directions to reduce bubble ratio compared to standard 1F1B.

New file: dualpipe_schedule.py

Implements the 8-step DualPipe algorithm as a drop-in replacement for Megatron's get_forward_backward_func() output:

• WeightGradStore – defers weight-gradient computation to dedicated "W" steps, enabling zero-bubble scheduling
• _split_data_iterator – pre-buffers and splits a single data iterator into two halves (one per pipeline direction) without upstream data-pipeline changes
• forward_backward_dualpipe() – the schedule itself; same keyword interface as Megatron's built-in schedules

Model building (training.py)

When --use-dualpipe is active each rank is allocated two model chunks:

• chunk[0] at pipeline position pp_rank (forward direction, rank 0 → N-1)
• chunk[1] at pipeline position N-1-pp_rank (mirror direction, rank N-1 → 0)

A new _fs_get_forward_backward_func() helper selects the DualPipe schedule or falls back to Megatron's standard selector — no change in behaviour when the flag is absent.

Configuration flag (arguments_fs.py)

--use-dualpipe with validation:

• Even pipeline_model_parallel_size > 1
• Incompatible with VPP (--num-layers-per-virtual-pipeline-stage) and --use-dualpipev
• Requires --untie-embeddings-and-output-weights
• Even num_microbatches ≥ pipeline_model_parallel_size × 2

Usage

training:
  pipeline_model_parallel_size: 8   # must be even
  use_dualpipe: true
  untie_embeddings_and_output_weights: true
  global_batch_size: 512            # num_microbatches must be even and >= pp_size*2

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}