feat: add dry-run mode to offline generators and improve docs

[aydnOktay]

This PR adds a --dry-run mode to both atropos-sft-gen and atropos-dpo-gen and updates the docs to make offline data generation and Windows setup easier.

What’s included

• New --dry-run mode for offline generators

  • atropos-sft-gen gains an sft_dry_run helper that:
    • loads the configured tokenizer, and
    • checks connectivity to the rollout server via GET /batch,
    • without registering a run or writing any output files.
  • atropos-dpo-gen gains a symmetric dpo_dry_run helper with the same behavior.
  • Both CLIs accept a --dry-run flag to trigger these checks:
    • atropos-sft-gen ... --dry-run
    • atropos-dpo-gen ... --dry-run

• More actionable error messages

  • Clear guidance when:
    • the tokenizer cannot be loaded (wrong model name / missing HF access),
    • the rollout server is unreachable,
    • or it returns an unexpected HTTP status code for GET /batch.

• Unit tests for the new behavior

  • atroposlib/tests/test_offline_cli_dry_run.py verifies:
    • sft_dry_run / dpo_dry_run call AutoTokenizer.from_pretrained and hit the API,
    • the CLI entrypoints call the dry-run helpers when --dry-run is passed instead of running a full data grab.

• Documentation improvements

  • README.md:
    • Extends the Offline Data Generation Quick Start section with an optional --dry-run step before collecting SFT data.
    • Adds a Windows Quickstart subsection with PowerShell examples for:
      • creating & activating a virtualenv,
      • starting run-api and an example environment,
      • running atropos-sft-gen with --dry-run and then generating data.
    • Clarifies the Windows virtualenv activation command in the troubleshooting section.

Why

• Offline SFT/DPO runs can be expensive; users typically want to confirm that:
  • the tokenizer name is valid, and
  • the rollout server is reachable and correctly configured
    before writing large datasets.
• A dedicated --dry-run mode shortens this feedback loop and avoids accidental mutations to the rollout server state.
• Windows users previously had to mentally translate POSIX shell examples; providing copy‑pasteable PowerShell snippets reduces friction for new contributors.

How to use

SFT dry-run

atropos-sft-gen path/to/output.jsonl
--tokenizer Qwen/Qwen2.5-1.5B-Instruct
--dry-run

DPO dry-run

atropos-dpo-gen path/to/output.jsonl
--tokenizer Qwen/Qwen2.5-1.5B-Instruct
--dry-run