Lazily load payloads from environments

[BarceloChristian]

USD provides the notion of payloads as lazily loaded references that.
In the context of huge photo-realistic environments as the ones Isaac Sim shines the most with, having to open the scenario to then disable the payloads if needed.
Instead, adding the option to open a USD with ALL payloads disabled would improve the bring up time a lot and ease the edition of highly composited environments.

This could be added directly to Kit (or an Isaac Sim's extension) with a menu in File -> Partial Load (or any other name) triggering a piece of code similar to this:

import omni.usd
from pxr import Usd
stage_path = "/path/to/my/stage.usd"

context = omni.usd.get_context()
context.open_stage(stage_path, None, load_set=omni.usd.UsdContextInitialLoadSet.LOAD_NONE)

If we were to go even one more step forward, we could allow to select which prim paths to load or not. Even tho a little more convoluted, it may be useful

import omni.usd
from pxr import Usd

stage_path = "/path/to/my/stage.usd"
subtree_prim_path_to_keep_unloaded = "/World/heavy_tree_models"

rules = Usd.StageLoadRules()
rules.AddRule("/", Usd.StageLoadRules.AllRule) # Load all!
rules.AddRule(subtree_prim_path_to_keep_unloaded, Usd.StageLoadRules.NoneRule) # But not this subtree

context = omni.usd.get_context()
context.open_stage(
    stage_path, 
    load_set=omni.usd.UsdContextInitialLoadSet.LOAD_NONE
) # This opens the stage with all the payloads unloaded!

stage = context.get_stage()
stage.SetLoadRules(rules) # Set the load rules right after open_stage, it will load any payload that it should.