changes on plant

[zeshunzong]

This change is 

[zeshunzong]

I think there is some lint issue with the old multibody_plant.h /cc that results in a ton of changes and I don't see a way to get rid of this.
But essentially only several lines are modified,
in multibody_plant.cc: https://github.com/zeshunzong/drake/blob/changes-made-to-plant/multibody/plant/multibody_plant.cc#L2776-L2790
in multibody_plant.h: https://github.com/zeshunzong/drake/blob/changes-made-to-plant/multibody/plant/multibody_plant.h#L5145-L5186

+@xuchenhan-tri

Reviewable status: 0 of 16 files reviewed, all discussions resolved (waiting on @xuchenhan-tri)

[xuchen-han]

multibody/plant/multibody_plant.h line 5161 at r1 (raw file):

    }

    // ----------- below is rigid part --------------

This comment is ill informed. Everything below this line is skipped for your purpose. if (discrete_update_manager_) is always true for discrete sims and your mpm sim is always discrete.

[xuchen-han]

Please only PR things that are ready for review. That means that unrelated whitespace changes and code that's not yet ready for review shouldn't be part of the PR.

I looked at the section you highlighted. They look fine to me, except one comment that misinformed.

Reviewable status: 0 of 16 files reviewed, 1 unresolved discussion (waiting on @zeshunzong)