Need to reduce the number of multi-node jobs per workflow run

[yhtang]

There are currently 200 backlog jobs in the CI cluster.

There are two options that we can implement:

• Merge jobs using the same image so that we don't re-creating the enroot containers repeatedly.
• Conditionally trigger MGMN jobs so that, e.g., changes only related to one model does not cause jobs for another model to run.

[yhtang]

Closing as the problem is no longer relevant.