Make example_id unique in EnvGroup

[mikasenghaas]

Description

prime-rl depend on example_id being unique across the entire env dataset - this is currently not the case for EnvGroup.

from datasets import Dataset

import verifiers as vf

a = vf.SingleTurnEnv(
    dataset=Dataset.from_dict({"question": ["a"], "completion": ["A"]})
)
b = vf.SingleTurnEnv(
    dataset=Dataset.from_dict({"question": ["a"], "completion": ["A"]})
)

print(vf.EnvGroup([a, b]).get_dataset().to_list())

Previously, both samples have ID 0

[{'question': 'a', 'completion': 'A', 'example_id': 0, 'prompt': [{'content': 'a', 'role': 'user'}], 'task': 'env_0'}, {'question': 'a', 'completion': 'A', 'example_id': 0, 'prompt': [{'content': 'a', 'role': 'user'}], 'task': 'env_1'}]

Now, they have IDs 0 and 1, as expected

[{'question': 'a', 'completion': 'A', 'prompt': [{'content': 'a', 'role': 'user'}], 'task': 'env_0', 'example_id': 0}, {'question': 'a', 'completion': 'A', 'prompt': [{'content': 'a', 'role': 'user'}], 'task': 'env_1', 'example_id': 1}]

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes