logger: log to summary first, then to logs

[dberenbaum]

Extracted from #142 (review).

Would summary -> tsv be more helpful (this would obviously require summary to always exist)? It's more intuitive to me (and follows the internal logic of MetricLogger._metrics) to gather all metrics for a step and then append to metrics logs. It also enables no-step scenarios like classical ML algorithms by logging the summary without ever creating the tsv files.

[daavoo]

Related: #157 (comment)

[daavoo]

Coming from #157 , looks like instead of switching tsv -> summary order would be better to just do tsv and summary in the same place (dvclive.log)

[dberenbaum]

From #157 (comment):

@dberenbaum @pared even if we consider thesummary behavior fixed (by moving it to log), it doesn't feel like there is a way to fulfill the 3 points listed above when considering html and checkpoints.

This might not be an issue since I think a final checkpoint is implicitly added at the end if anything has changed since the last checkpoint.

[pared]

@dberenbaum I modified the discussion title to make it clearer