fix trainer epcoh logging

TODO.md

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+# Long-term
+- Implement a dataloader in-house, with a clear, lightweight mechanism for
+  collection-of-structures to structure-of-collections. For multi-proc handling
+  (happens in torch's dataloader, as well as the BatchedDataset for two
+  different purposes), we should rely on (a hopefully more stable) `execlib`.
+- `Domains` may be externalized (`co3` or `convlib`)
+- Up next: CLI, fully JSON-ification of model selection + train.
+- Consider a "multi-train" alternative (or arg support in `train()`) for
+  training many "rollouts" from the same base estimator (basically forks under
+  different seeds). For architecture benchmarking above all, seeing average
+  training behavior. Consider corresponding `Plotter` methods (error bars)