Mark test_lstm_packed as slow (#107048)
The test takes >30 minutes to run on some configurations and keeps getting unmarked as slow by the automatic slow test detection.
Examples:
https://ossci-raw-job-status.s3.amazonaws.com/log/15824750763
https://ossci-raw-job-status.s3.amazonaws.com/log/15802766247
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107048
Approved by: https://github.com/huydhn
