fix: lazy-import heavy dependencies in CLI to fix commands without backend extras (#571)

`art install-skills`, `art train-sft`, `art train-rl`, and `art migrate`
all crash with ModuleNotFoundError when backend extras (torch, fastapi,
uvicorn, numpy) are not installed, even though they don't need them.

- Move fastapi, uvicorn, pydantic, LocalBackend, and other backend-only
  imports from top-level cli.py into the `run` command where they're
  actually used
- Remove unused imports (Optional, Provider, TogetherDeploymentConfig,
  WandbDeploymentConfig)
- Lazy-import calculate_step_std_dev in model.py to break the
  __init__ -> utils -> model -> numpy eager import chain

Co-authored-by: Claude Opus 4.6 <[email protected]>

Author: Kovbo

src/art/cli.py

@@ -1,26 +1,10 @@
 import json
 from pathlib import Path
 import socket
-from typing import Any, AsyncIterator, Optional
+from typing import Any, AsyncIterator
 
 from dotenv import load_dotenv
-from fastapi import Body, FastAPI, Request
-from fastapi.responses import JSONResponse, StreamingResponse
-import pydantic
 import typer
-import uvicorn
-
-from . import dev
-from .errors import ARTError
-from .local import LocalBackend
-from .model import Model, TrainableModel
-from .trajectories import TrajectoryGroup
-from .types import TrainConfig
-from .utils.deployment import (
-    Provider,
-    TogetherDeploymentConfig,
-    WandbDeploymentConfig,
-)
 
 load_dotenv()
 
@@ -302,6 +286,18 @@ def migrate(
 def run(host: str = "0.0.0.0", port: int = 7999) -> None:
     """Run the ART CLI."""
 
+    from fastapi import Body, FastAPI, Request
+    from fastapi.responses import JSONResponse, StreamingResponse
+    import pydantic
+    import uvicorn
+
+    from . import dev
+    from .errors import ARTError
+    from .local import LocalBackend
+    from .model import Model, TrainableModel
+    from .trajectories import TrajectoryGroup
+    from .types import TrainConfig
+
     # check if port is available
     def is_port_available(port: int) -> bool:
         with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:

src/art/model.py

@@ -15,7 +15,6 @@
 from .costs import CostCalculator
 from .trajectories import Trajectory, TrajectoryGroup
 from .types import TrainConfig, TrainSFTConfig
-from .utils.old_benchmarking.calculate_step_metrics import calculate_step_std_dev
 from .utils.trajectory_logging import write_trajectory_groups_parquet
 
 if TYPE_CHECKING:
@@ -638,6 +637,10 @@ def _add_costs(metrics_dict: dict[str, float | int | bool]) -> None:
                 averages[f"group_metric_{metric}"] = sum(values) / len(values)
 
         # Calculate average standard deviation of rewards within groups
+        from .utils.old_benchmarking.calculate_step_metrics import (
+            calculate_step_std_dev,
+        )
+
         averages["reward_std_dev"] = calculate_step_std_dev(trajectory_groups)
 
         # Merge in any additional metrics passed directly