Add gracefully stop time for step to collect logs (#2877)
Add gracefully stop time for step to collect logs (#2877)

* Collect step's output to given file

* add SQLFLOW_WORKFLOW_STEP_LOG_FILE evn to control log path

* Add graceful exit time for step
sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow