Add loading screen for train/play, default newton and rsl_rl across core

[StafaH]

Description

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 added a changelog fragment under source/<pkg>/changelog.d/ for every touched package (do not edit CHANGELOG.rst or bump extension.toml — CI handles that)
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Greptile Summary

The PR adds an interactive startup screen around train/play initialization and allows unified entrypoints to infer an RL backend from task registration. It also changes the shipped core-task defaults to Newton MJWarp and the Newton renderer.

• Adds staged progress, activity reporting, startup-output spooling, and run summaries.
• Integrates the screen across all supported RL train/play backends.
• Adds task-aware default RL-library resolution.
• Switches core task physics and renderer defaults from Isaac Sim PhysX/RTX to Newton.

Confidence Score: 5/5

The PR appears safe to merge with no concrete changed-code defects identified.

The loading-screen lifecycle, backend dispatch fallback, inherited task presets, and coordinated Newton defaults have reachable implementations consistent with their documented startup flows, and the security lead concerned unchanged pre-existing parsing behavior.

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/app/loading_screen.py	Adds the loading-screen lifecycle, staged progress display, activity stack, output spooling, and console restoration.
source/isaaclab_rl/isaaclab_rl/entrypoints/common.py	Adds shared startup-screen construction and run-summary formatting across RL backends.
source/isaaclab_rl/isaaclab_rl/entrypoints/dispatch.py	Makes explicit RL-library selection optional when a task’s registered configurations provide a default.
source/isaaclab_tasks/isaaclab_tasks/utils/parse_cfg.py	Adds registry-based RL-library resolution with an RSL-RL fallback.
source/isaaclab_tasks/isaaclab_tasks/utils/presets.py	Changes the multi-backend renderer’s default from Isaac RTX to the Newton renderer.
source/isaaclab/isaaclab/utils/timer.py	Extends timed contexts with optional loading-screen activity reporting.

Sequence Diagram

sequenceDiagram
    participant CLI as Unified train/play CLI
    participant Registry as Task registry
    participant Screen as LoadingScreen
    participant Launcher as Simulation launcher
    participant Env as Isaac Lab environment
    participant Agent as RL backend

    CLI->>Registry: Resolve task and default RL library
    CLI->>Screen: Open and show run summary
    Screen->>Screen: Redirect startup output
    CLI->>Launcher: Launch selected simulation backend
    Launcher->>Screen: Report startup activities
    CLI->>Env: Create environment
    Env->>Screen: Report scene, physics, and manager setup
    CLI->>Agent: Build wrapper and policy
    CLI->>Screen: Close and restore console
    CLI->>Agent: Begin training or playback

Loading

_{Reviews (1): Last reviewed commit: "Make startup more detailed, add logo" | Re-trigger Greptile}

[isaaclab-review-bot]

Isaac Lab Review Bot

The loading-screen integration, activity reporting, optional RL-library resolution, and Newton-default migration are broadly coherent. Before merge, add the required isaaclab_newton changelog fragment and ensure a task’s marked default RL library is validated against the supported libraries before dispatch.

• Design and architecture: The loading screen is cleanly scoped to startup, with output spooling and replay on failure, while the global report_activity hook keeps instrumentation decoupled from callers. The backend resolver has one boundary issue: registry metadata is trusted without checking that the selected library is supported by the requested action, allowing dispatch to fail with an uncaught KeyError.
• API: LoadingScreen, format_run_summary, and report_activity are exported and documented, and Timer(activity=...) is an additive keyword-compatible extension. resolve_rl_library should uphold its documented selection boundary by returning a marked default only when it appears in the supplied libraries collection; otherwise it should fall back or return None.
• Implementation: The screen lifecycle, progress handling, ticker shutdown, and file-descriptor restoration are internally consistent. The isaaclab_newton Timer instrumentation also requires a changelog fragment for that touched package under the repository’s per-package changelog rule.

Minor fixes needed. Posted 2 actionable findings inline.

Automated review; human maintainers own approval decisions.

[isaaclab-review-bot]

🟡 Warning · Implementation — Missing changelog fragment for isaaclab_newton

This PR modifies source/isaaclab_newton/ (Timer activity labels here and at lines 2010, 2081) but adds no fragment under source/isaaclab_newton/changelog.d/, while isaaclab, isaaclab_rl, and isaaclab_tasks each received one. Repository rules require one fragment per touched package so the nightly job can compile the changelog and version bump. Add <slug>.rst, or <slug>.skip if these labels are considered non-user-facing.

[isaaclab-review-bot]

🟡 Warning · Api — Marked RL library bypasses libraries filter

resolve_rl_library documents that it returns a name from libraries or None, but the default_rl_library branch returns the registered value unchecked. dispatch.run_cli then evaluates backends[library], so a task marking a library that is not a supported backend for the requested action raises an uncaught KeyError instead of the documented fallback or the clean "argument required" exit. Validate the marker against libraries before returning it.