logger: log to summary first, then to logs #145
Description
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.