[ci] only upload summarized test stats for PRs (#80295)
We are uploading too many tests which is stressing out Rockset. We do
not really use granular per-test statistics for PRs, so change to
uploading summary information for PRs.
We will still be uploading per-test stats from master.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80295
Approved by: https://github.com/seemethere, https://github.com/malfet
