PyTorch: [torchci] Remove the report of a session that starts no test (#201118)

Summary: A pytest session that never starts a test, such as run_test.py's serial pass over a file without serial tests, now removes its report when it ends. Such a report holds only the report line, so it adds no rows, but each one is still uploaded and ingested; removing them cuts that ingest traffic. The report is kept whenever a test started, so the crash and timeout handling from #200942 is unaffected: run_test.py only appends a run the writer published when its test started.