[Feature Request] Track and shrink Hydra Config/class kwarg parity gaps

Motivation

This is a tracking issue, not a request to close the ~60 Hydra Config / class kwarg-parity gaps in one PR.

Agents.md §14 (Config / class parity) says every wrapped-class __init__ kwarg must appear on the matching *Config with the same default, be popped in the factory, and be forwarded. A missing field is dropped silently by Hydra: the user thinks they set it, the constructor never sees it.

test/test_configs.py already encodes that rule, but it also carries a large allowlist and deliberately does not check default-value equality. Those gaps should be tracked here and closed one area at a time.

Solution

Keep this issue as the tracker. Confirm the process below (or amend it):

1. Leave _CONFIG_PARITY_KNOWN_GAPS in place as the working list.
2. One follow-up PR per area (replay buffers, collectors, transforms, env libs, …). Each PR adds the missing Config fields, forwards them, and removes only those names from the allowlist.
3. Turn on default-equality asserts for an area once its names are gone from the allowlist.
4. Do not dump the entire allowlist into a single PR.

Alternatives

• Close the allowlist in one PR. That mixes unrelated modules, is hard to review, and is the opposite of what this issue is for.
• Drop the parity test. That would make the silent-Hydra-drop problem worse.

Additional context

Allowlist (62 names) in test/test_configs.py:152-217:

_CONFIG_PARITY_KNOWN_GAPS = frozenset(
    {
        "ActionMaskConfig",
        "AggregatorConfig",
        ...
        "MultiSyncCollectorConfig",
        ...
        "PrioritizedSliceSamplerConfig",
        ...
        "ReplayBufferConfig",
        ...
        "VecNormConfig",
        "VecNormV2Config",
        "WriterConfig",
    }
)

Default equality is explicitly not enforced (test/test_configs.py:278-285):

    - Only field-name presence is checked; default-value *equality* between a
      Config field and the corresponding ``__init__`` kwarg is NOT enforced, so
      a Config default that drifts from the constructor's default still passes.
    - Wrapped ``__init__`` signatures made up purely of ``*args``/``**kwargs``
      expose no named parameters to diff, so their configs pass vacuously, and
      a ``**kwargs`` catch-all next to named parameters hides any kwarg that is
      only reachable through it.

Concrete gaps already visible in this checkout:

Replay buffers. ReplayBuffer.__init__ accepts transport and transport_options (torchrl/data/replay_buffers/replay_buffers/base.py:400-401). ReplayBufferConfig (torchrl/trainers/algorithms/configs/data.py:399-424) omits both. A Hydra user setting transport: ray has no effect.

Collectors. CollectorConfig exposes auto_register_policy_transforms, pre_collect_hook, post_collect_hook, compact_obs (collectors.py:79-82). MultiCollector.__init__ accepts the same four (torchrl/collectors/_multi_base.py:472-475). MultiSyncCollectorConfig (collectors.py:151-198) and MultiAsyncCollectorConfig (collectors.py:212-263) omit all four.

*args, **kwargs blind spot. MultiAsyncCollector.__init__ is (torchrl/collectors/_multi_async.py:131-132):

def __init__(self, *args, **kwargs):
    super().__init__(*args, **kwargs)

The parity test therefore cannot see MultiCollector's kwargs and MultiAsyncCollectorConfig is not even on the allowlist. It can pass while missing the same fields as MultiSyncCollectorConfig.

The test docstring (test/test_configs.py:294-297) already says the allowlist should only shrink.

Checklist

• I have checked that there is no similar issue in the repo (required)