[Bug] Latest mitsuba3 failed for prbvolpath

[andyyankai]

The latest version of Mitsuba3 using the new version of DrJit failed when using prbvolpath for backward/forward mode on heterogeneous volumes with NEE.

It seems the issue is caused by performing backward/forward operations in a loop nested within another loop during tracing, which might be related to the latest version of DrJit. Here is the error message:

Traceback (most recent call last): File "C:\xxx\mitsuba\python\ad\integrators\prbvolpath.py", line 271, in _loop_body dr.backward(δL * contrib) RuntimeError: CustomOp::backward(): the reverse-mode derivative of a complex loop (with max_iterations != -1) is not yet implemented!
To reproduce the issue, you can run the code using python validation_heter.py. I have attached a file to help with this: validation_heter.zip.

The issue can be triggered by backpropagating a loss for sigma_t for any heterogeneous volume when using prbvolpath. I have also included a prbvolpath_m1 that supports forward_mode AD in the zip.

Thank you!