Prompt to install policy extras (smolvla/pi0/diffusion) before training starts

[nobullryder]

Problem

Training a policy that pulls an optional LeRobot extra fails with a buried ImportError — but only after the training subprocess has already started, so the user just sees the run die in the logs:

ImportError: 'transformers' is required but not installed.
Install it with: pip install 'lerobot[smolvla]'

smolvla, pi0, and pi0_fast need transformers; diffusion needs diffusers.

What this does

A pre-flight check on Start Training, built entirely on the existing install-extra machinery (InstallManager, useInstallExtra, InstallProgress, WandbInstallDialog) — no new infrastructure.

• GET /system/policy-extra/{policy_type} does a live importlib.util.find_spec probe and returns the install target. POST .../install + GET .../install-status reuse InstallManager to run pip install lerobot[<extra>] in the background.
• On Start Training the UI pre-flights the endpoint; if the extra is missing it opens a PolicyExtraDialog (a parameterized sibling of WandbInstallDialog) for a one-click install instead of starting a run that's doomed to crash.

Policy	Probe	Install
smolvla	transformers	lerobot[smolvla]
pi0, pi0_fast	transformers	lerobot[pi]
diffusion	diffusers	lerobot[diffusion]
act, vqbet, tdmpc, sac, …	—	reported available (no-op)

Tests

tests/test_utils_system.py covers the policy→extra mapping, the find_spec availability logic, the core-policy no-op, the install no-op, and the GET /system/policy-extra/{policy_type} route.

Notes

• After installing, restart LeLab so the new package is importable (same as the existing wandb/accelerate extras).
• Generated frontend/dist is included (built with npm ci against the committed lockfile, matching the build_frontend workflow) — please review the source files first.

🤖 Generated with Claude Code