Icevision: Icevision records metrics as a dict; in fastai, a scalar would provide better integration

[adamfarquhar]

In FastAI, various callbacks can access and use recorded metrics. For example, the SaveModelCallback can save the model in the case that it is better than the previous best. It looks a the values in the learner's recorder for this information (learn.recorder.values). Normally, it expects to see a scalar value, but icevision appears to stash a dict there.

For example, when using the cocometrics, learn.recorder.values[0][2] is a dict with a single key - {'mAPI' : 0.1234}. If the metrics were stashed directly, then the additional machinery would work. This would require a little additional adaptation to provide the metric name as well.

This callback can be used as-is, but only with valid_loss, training_loss rather than the metric (which is better aligned with the model's task performance).

[lgvaz]

Hey Adam, this issue makes a very good point on why we should not use dicts as metrics for fastai, I'm on board with that.

The current implementation (using dicts) was always a quick fix in my mind, the better solution would be to convert each of the items in that dict to a separate fastai metric. Help from fastai experts would be appreciated here =)

What we need to do is write an Adapter that handles all of this, for instance, this is how the current adapter for fastai looks like:

class FastaiMetricAdapter(fastai.Metric):
    def __init__(self, metric: Metric):
        self.metric = metric

    def reset(self):
        pass

    def accumulate(self, learn: fastai.Learner):
        self.metric.accumulate(records=learn.records, preds=learn.converted_preds)

    @property
    def value(self) -> Dict[str, float]:
        return self.metric.finalize()

    @property
    def name(self) -> str:
        return self.metric.name

Note that this subclasses fastai.Metric, the point of issue here is in def value, fastai expects a float to be returned, but we're returning a dict.

To be clear, we should not change the return value of metric.finalize() (this is the icevision implementation) but instead how to write an adapter that handles this difference (converting a dict of values to multiple fastai.Metric instances.

[lgvaz]

A hotfix was implemented in #459, it only works when the metric returns a single value.

    @property
    def value(self) -> Dict[str, float]:
        logs = self.metric.finalize()
        return next(iter(logs.values()))

[FraPochetti]

Hey guys, can you please assign this one to me?

[FraPochetti]

As discussed on Discord, I think this would be a better match for me right now!
Just for sake of clarity, I will _unassign_ myself.