Add option of mitsuba parallel loading.

[chaihahaha]

Fix strange assertion error caused by mitsuba when load_scene is called many times because of this unresolved issue: mitsuba-renderer/mitsuba3#849

Description

This PR added a new option to turn off the parallel loading in mitsuba.

• Fixes a bug?

This PR will mitigate the problem caused by mitsuba issue (mitsuba-renderer/mitsuba3#849) when the scene is repeatedly loaded for many times.

• Adds a new feature?

This PR added a new feature to turn on/off the parallel loading scene behavior.

• Introduces API changes?

This is needed because loading scene in parallel is a behavior that has to be tuned on some machines.

• Other contributions

This submission is to add an option to switch a loading behavior that is previously hidden .

Checklist

[x] Detailed description
[x] Added references to issues and discussions
[ ] Added / modified documentation as needed
[ ] Added / modified unit tests as needed
[ ] Passes all tests
[x] Lint the code
[x] Performed a self review
[x] Ensure you Signed-off the commits. Required to accept contributions!
[ ] Co-authored with someone? Add Co-authored-by: user@domain and ensure they signed off their commits too.

[merlinND]

Hello @chaihahaha,

Thank you for this pull request! We may indeed want to have this option exposed in the API.

About the issue itself, could you try again after upgrading Mitsuba to version 3.4.1 and let us know if you still experience the remain == 1 problem?

[chaihahaha]

@merlinND I tested with the following code after upgrading mitsuba, the asserttion failed... remain == 1 persists.

# pip install mitsuba==3.4.1

import sionna

for _ in range(10000):
    m = sionna.rt.load_scene(sionna.rt.scene.munich)
    del m

[merlinND]

Hello @chaihahaha,

I cannot reproduce the remain == 1 error with mitsuba==3.5.0, drjit==0.4.4.
Are you still encountering the crash with these latest versions? If so, could you please rebase your PR so that we can merge it?