Add dataset card for A2-OPD

This PR adds a dataset card for the A2-OPD dataset. It includes the appropriate license, task category, and tags, links to the paper and code repository, a description of the dataset structure, and a sample usage snippet for downloading the data.

README.md

@@ -0,0 +1,39 @@
+---
+license: apache-2.0
+task_categories:
+- text-generation
+tags:
+- math
+- code
+- on-policy-distillation
+---
+
+# A2-OPD Dataset
+
+This dataset accompanies the paper [Rethinking Relative Policy Differences Transfer of On-Policy Distillation](https://huggingface.co/papers/2610.13205).
+
+Code: https://github.com/KnowledgeXLab/A2-OPD
+
+The dataset is organized by `math/` and `code/` folders and contains training and evaluation data for the A2-OPD experiments.
+
+## Dataset Structure
+
+| Path | Split | Used for |
+|------|-------|----------|
+| `math/train.parquet` | train | DeepMath-103K training |
+| `math/test.parquet` | test | Training validation and offline math eval |
+| `code/train.parquet` | train | Eurus-RL-Code training |
+| `code/validation.parquet` | validation | Training validation during code RL |
+| `code/benchmarks/Humaneval_Plus/HumanEvalPlus.jsonl` | test | Offline HumanEval+ |
+| `code/benchmarks/Mbpp_Plus/MbppPlus.jsonl` | test | Offline MBPP+ |
+| `code/benchmarks/LivecodeBench/test.jsonl` | test | Offline LiveCodeBench |
+
+Training data must be in `verl` parquet format. Each row needs fields such as `data_source`, `prompt`, `ability`, `reward_model.ground_truth`, and optional `extra_info`.
+
+## Sample Usage
+
+```bash
+# One-click download (requires huggingface_hub: `hf` or `huggingface-cli`)
+hf download KnowledgeXLab/A2-OPD --repo-type dataset --local-dir data
+export A2OPD_DATA_ROOT=$PWD/data   # default layout matches training/eval scripts
+```