Add a flag to benchmarks script to keep the test report directory (#96398)
I notice from the Rockset data that there are only `float32` records, while there should be both dtypes there. It turns out that the benchmarks script generated by `runner.py` always removes the output directory by default, so there are only records from `float32` running later left.
For example, `rm -rf /var/lib/jenkins/workspace/test/test-reports` appeared twice in the CI log https://ossci-raw-job-status.s3.amazonaws.com/log/11840774308.
I'm adding a new flag `--keep-output-dir` to keep the output directory. This is off by default as I'm not sure how this script is used internally, people probably expect to see the output directory cleaned up everytime.
### Testing
Not really want to start the 10h jobs just to test this small flag, so I'm triple check the change to make sure that there is no bug
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96398
Approved by: https://github.com/weiwangmeta
