refactor: emit strategy metrics as floats

- Converts detached strategy metrics before returning loss output
- Removes redundant item conversion from the trainer loop
- Updates the documented contract and regression tests

astrai/trainer/strategy.py

@@ -15,7 +15,7 @@ from astrai.trainer.rollout import RolloutResult
 
 class LossOutput(TypedDict):
     loss: Tensor
-    metrics: Dict[str, Tensor]
+    metrics: Dict[str, float]
 
 
 class LogprobsOutput(TypedDict):
@@ -148,14 +148,14 @@ class BaseStrategy(ABC):
         metrics["loss"] = total_loss
         return {
             "loss": total_loss,
-            "metrics": {name: value.detach() for name, value in metrics.items()},
+            "metrics": {name: value.detach().item() for name, value in metrics.items()},
         }
 
     @staticmethod
     def _normalize_output(output: Union[LossOutput, Tensor]) -> LossOutput:
         if isinstance(output, dict):
             return output
-        return {"loss": output, "metrics": {"loss": output.detach()}}
+        return {"loss": output, "metrics": {"loss": output.detach().item()}}
 
     def supports_online(self) -> bool:
         """Whether this strategy can operate with a rollout runner.

astrai/trainer/trainer.py

@@ -84,10 +84,7 @@ class Trainer:
                         self._call_callbacks("on_batch_begin", context)
                         loss_output = context.strategy(batch)
                         context.loss = loss_output["loss"].item()
-                        context.metrics = {
-                            name: value.item()
-                            for name, value in loss_output["metrics"].items()
-                        }
+                        context.metrics = loss_output["metrics"]
                         stand_loss = loss_output["loss"] / executor.grad_accum_steps
                         executor.backward(stand_loss)
                         context.consumed_samples += (

docs/guides/training.md

@@ -89,7 +89,7 @@ on_train_end
 
 Default callbacks (in order): `gradient_checkpointing` (activation checkpointing, optional), `checkpoint` (safetensors, rank-0), `metric` (JSONL + validation, rank-0), `progress_bar` (tqdm), `gradient_clipping` (always registered; computes grad norm, clips only when `max_grad_norm` is not `None`).
 
-Strategies return `{"loss": Tensor, "metrics": Dict[str, Tensor]}` when called by the trainer. Built-in metrics include the task-specific loss and, for MoE models, `moe_aux_loss` plus `moe_aux_loss_weighted`. Direct `compute_loss(batch)` calls continue to return a single loss tensor.
+Strategies return `{"loss": Tensor, "metrics": Dict[str, float]}` when called by the trainer. Built-in metrics include the task-specific loss and, for MoE models, `moe_aux_loss` plus `moe_aux_loss_weighted`. Direct `compute_loss(batch)` calls continue to return a single loss tensor.
 
 ## Strategies
 

tests/trainer/test_loss_output.py

@@ -1,5 +1,6 @@
 from types import SimpleNamespace
 
+import pytest
 import torch
 
 from astrai.model.transformer import AutoRegressiveLM
@@ -33,16 +34,14 @@ def test_seq_strategy_combines_and_reports_moe_aux_loss(device):
         "moe_aux_loss",
         "moe_aux_loss_weighted",
     }
-    torch.testing.assert_close(
-        output["loss"],
+    assert output["loss"].item() == pytest.approx(
         output["metrics"]["task_loss"] + output["metrics"]["moe_aux_loss_weighted"],
     )
-    torch.testing.assert_close(
-        output["metrics"]["moe_aux_loss_weighted"],
+    assert output["metrics"]["moe_aux_loss_weighted"] == pytest.approx(
         0.25 * output["metrics"]["moe_aux_loss"],
     )
     assert output["loss"].requires_grad
-    assert all(not metric.requires_grad for metric in output["metrics"].values())
+    assert all(isinstance(metric, float) for metric in output["metrics"].values())
 
 
 def test_metric_callback_includes_dynamic_strategy_metrics(tmp_path):
@@ -103,4 +102,4 @@ def test_legacy_strategy_tensor_loss_is_normalized():
     output = strategy({})
 
     assert output["loss"].item() == 2.0
-    assert output["metrics"]["loss"].item() == 2.0
+    assert output["metrics"]["loss"] == 2.0