I was reading the Axon.Loop.metric/5 documentation (Axon.Loop — Axon v0.5.1) and it references `:mean_squared_error`

```
loop
|> Axon.Loop.metric(:mean_squared_error, "Error") # Will be overwritten
|> Axon.Loop.metric(:mean_absolute_error, "Error") # Will be used
```

It stood out because I remember the metric should be:

`metric`

must be an atom which matches the name of a metric in`Axon.Metrics`

, or an arbitrary function which returns a tensor or container.

And it does not exist in `Axon.Metrics`

. I’m assuming `:mean_squared_error`

was not an arbitrary function otherwise it would have been written like `:custom_metric`

(or similar). So, just wondering if maybe it was lost or deliberately removed?