Lazifies IsaacLab import system

[ooctipus]

Description

This PR improves lazy-loading and config-only import behavior so environment config construction stays backend-free and deterministic.

• Simplifies and tightens cascading namespace resolution in isaaclab.utils.module.attach_cascading with strict fail-fast imports.
• Prevents eager callable resolution during deepcopy by adding __copy__ / __deepcopy__ behavior in ResolvableString.
• Reorders isaaclab.sim.utils cascading lookup to prefer lightweight modules first.
• Fixes locomanipulation pick-place MDP namespace/export boundaries (terminations) and removes a backend-heavy module-level import path in pick-place terminations.
• Updates changelog for the lazy-loader/config-import cleanup.

Fixes # (issue)

Type of change

• New feature (non-breaking change which adds functionality)

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[AntoineRichard]

I'm a bit concerned about user friendlyness of this PR. This looks inelegant, and not very user friendly.

import lazy_loader as lazy

__getattr__, __dir__, __all__ = lazy.attach(
    __name__,
    submod_attrs={
        "actuator_base": ["ActuatorBase"],
        "actuator_base_cfg": ["ActuatorBaseCfg"],
        "actuator_net": ["ActuatorNetLSTM", "ActuatorNetMLP"],
        "actuator_net_cfg": ["ActuatorNetLSTMCfg", "ActuatorNetMLPCfg"],
        "actuator_pd": ["DCMotor", "DelayedPDActuator", "IdealPDActuator", "ImplicitActuator", "RemotizedPDActuator"],
        "actuator_pd_cfg": [
            "DCMotorCfg",
            "DelayedPDActuatorCfg",
            "IdealPDActuatorCfg",
            "ImplicitActuatorCfg",
            "RemotizedPDActuatorCfg",
        ],
    },
)

How about we do something like that:

# lazy_imports.py
from __future__ import annotations

from collections.abc import Iterable
import sys
import lazy_loader as lazy

def lazy_export(*imports: tuple[str, str | Iterable[str]]) -> None:
    """
    Usage in __init__.py:
        from .lazy_imports import lazy_export
        lazy_export(("foo", "A"), ("bar", ["B", "C"]))
    """
    caller_globals = sys._getframe(1).f_globals
    package_name = caller_globals["__name__"]

    submod_attrs: dict[str, list[str]] = {}
    for submod, names in imports:
        submod_attrs[submod] = [names] if isinstance(names, str) else list(names)

    __getattr__, __dir__, __all__ = lazy.attach(package_name, submod_attrs=submod_attrs)

    mod = sys.modules[package_name]
    mod.__getattr__ = __getattr__
    mod.__dir__ = __dir__
    mod.__all__ = __all__

This would simplify the code to:

from .lazy_imports import lazy_export

lazy_export(
    ("actuator_base", "ActuatorBase"),
    ("actuator_base_cfg", "ActuatorBaseCfg"),
    ("actuator_net", ["ActuatorNetLSTM", "ActuatorNetMLP"]),
    ("actuator_net_cfg", ["ActuatorNetLSTMCfg", "ActuatorNetMLPCfg"]),
    ("actuator_pd", ["DCMotor", "DelayedPDActuator", "IdealPDActuator", "ImplicitActuator", "RemotizedPDActuator"]),
    ("actuator_pd_cfg", ["DCMotorCfg", "DelayedPDActuatorCfg", "IdealPDActuatorCfg", "ImplicitActuatorCfg", "RemotizedPDActuatorCfg"]),
)

[AntoineRichard]

ooctipus#6 I have a PR that makes this a bit nicer.

[AntoineRichard]

This would likely solve #4742. But I'm wondering if we should have these imports inside our inits. What do we gain from these?