refactor(profiling): shrink lerobot_train.py diff via start()/finalize()

Replace the `with profiler or nullcontext():` wrap around the entire
training loop with explicit `profiler.start()` / `profiler.finalize()`
calls, and tighten `_section(...)` regions in `update_policy` to only
wrap the hot calls (forward / backward / optimizer.step).

This avoids ~120 lines of pure re-indentation noise while keeping the
exact same artifacts on disk and the same public behavior.

lerobot_train.py diff vs main: 267 -> 29 changed lines.

Made-with: Cursor

src/lerobot/scripts/lerobot_train.py

@@ -138,8 +138,7 @@ def update_policy(
             policy.parameters(), float("inf"), error_if_nonfinite=False
         )
 
-    with _section("optimizer"):
-        with lock if lock is not None else nullcontext():
+    with _section("optimizer"), lock if lock is not None else nullcontext():
         optimizer.step()
 
     optimizer.zero_grad()
@@ -327,6 +326,7 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
         profiler.record_deterministic_forward(
             policy=policy, dataset=dataset, batch_size=cfg.batch_size, preprocessor=preprocessor
         )
+        profiler.start()
 
     # Load precomputed SARM progress for RA-BC if enabled
     # Generate progress using: src/lerobot/policies/sarm/compute_rabc_weights.py
@@ -441,7 +441,7 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
         logging.info(
             f"Start offline training on a fixed dataset, with effective batch size: {effective_batch_size}"
         )
-    with profiler or nullcontext():
+
     for _ in range(step, cfg.steps):
         start_time = time.perf_counter()
         batch = next(dl_iter)

src/lerobot/utils/profiling_utils.py

@@ -356,14 +356,10 @@ class TrainingProfiler:
         if self._device.type == "cuda":
             torch.cuda.empty_cache()
 
-    def __enter__(self) -> TrainingProfiler:
+    def start(self) -> None:
         if self._device.type == "cuda":
             torch.cuda.reset_peak_memory_stats(self._device)
         self._torch_profiler.__enter__()
-        return self
-
-    def __exit__(self, *exc: Any) -> bool:
-        return self._torch_profiler.__exit__(*exc)
 
     @contextmanager
     def section(self, name: str) -> Iterator[None]:
@@ -394,6 +390,7 @@ class TrainingProfiler:
         self._torch_profiler.step()
 
     def finalize(self) -> None:
+        self._torch_profiler.__exit__(None, None, None)
         extra: dict[str, Any] = {"profile_mode": self._mode}
         if self._device.type == "cuda":
             extra["peak_memory_allocated_bytes"] = torch.cuda.max_memory_allocated(self._device)

tests/scripts/test_model_profiling.py

@@ -400,7 +400,7 @@ def test_training_profiler_section_records_duration(tmp_path):
         output_dir=tmp_path,
         device=torch.device("cpu"),
     )
-    with profiler:
+    profiler.start()
     with profiler.section("forward"):
         pass
     with profiler.section("backward"):