DrJit / nanobind memory leak warnings

[andrewgait]

Building the photon_emitter and custom_ptracer code against the current master version of Mitsuba3 was successful, and initial testing of scripts gives promising results.

However running a python script containing a photon_emitter and/or a custom_ptracer (I'm not sure which yet) leads to warning messages at the end of the script, for example:

jit_shutdown(): detected 1659 variable leaks:
 - float32 r24[1] = evaluated(), refs=2
 - float32 r25[1] = evaluated(), refs=2
 - float32 r26[1] = evaluated(), refs=1
 - float32 r27[1] = evaluated(), refs=1
 - float32 r28[1] = evaluated(), refs=1
 - float32 r29[1] = evaluated(), refs=1
 - float32 r30[1] = evaluated(), refs=1
 - float32 r33[1] = evaluated(), refs=2
 - float32 r40[1] = evaluated(), refs=2
 - uint32 r46[1] = evaluated(), refs=1
 - uint32 r54[1048576] = counter(), refs=4
 - uint64 r63[1048576] = add(r824, r820), refs=1
 - uint64 r67[1] = literal(), refs=2
 - float32 r79[1] = evaluated(), refs=2
 - float32 r81[1] = evaluated(), refs=2
 - float32 r82[1] = evaluated(), refs=2
 - float32 r83[1] = evaluated(), refs=2
 - float32 r84[1] = evaluated(), refs=2
 - float32 r85[1] = evaluated(), refs=2
 - float32 r87[1] = evaluated(), refs=2
 - float32 r89[1] = evaluated(), refs=8
 - float32 r90[1] = evaluated(), refs=8
 - float32 r91[1] = evaluated(), refs=8
 - float32 r104[1] = evaluated(), refs=2
 - float32 r106[1] = evaluated(), refs=2
 - float32 r111[1] = evaluated(), refs=2
 - float32 r125[1] = evaluated(), refs=2
 - float32 r146[1] = evaluated(), refs=2
 - float32 r157[1] = evaluated(), refs=2
 - float32 r165[1] = evaluated(), refs=2
 - float32 r166[1] = evaluated(), refs=2
 - float32 r169[1] = evaluated(), refs=2
 - float32 r171[1] = evaluated(), refs=8
 - float32 r173[1] = evaluated(), refs=8
 - float32 r174[1] = evaluated(), refs=8
 - float32 r179[1] = evaluated(), refs=2
 - float32 r183[1] = evaluated(), refs=2
 - float32 r217[1] = evaluated(), refs=2
 - float32 r218[1] = evaluated(), refs=2
 - float32 r219[1] = evaluated(), refs=2
 - float32 r220[1] = evaluated(), refs=2
 - float32 r221[1] = evaluated(), refs=2
 - float32 r223[1] = evaluated(), refs=8
 - float32 r233[1] = evaluated(), refs=8
 - float32 r234[1] = evaluated(), refs=8
 - float32 r238[1] = evaluated(), refs=2
 - float32 r242[1] = evaluated(), refs=2
 - float32 r246[1] = evaluated(), refs=2
 - float32 r289[1] = evaluated(), refs=4
 - float32 r290[1] = evaluated(), refs=4
 - (skipping remainder)
jit_registry_shutdown(): leaking 1 instances of type "mitsuba::Sensor".
jit_malloc_shutdown(): leaked
 - host-async memory: 48.02 MiB in 226 allocations
nanobind: leaked 2 instances!
 - leaked instance 0x7f280c06c540 of type "mitsuba.ProjectiveCamera"
 - leaked instance 0x7f27bc001550 of type "mitsuba.AdjointIntegrator"
nanobind: leaked 6 types!
 - leaked type "mitsuba.AdjointIntegrator"
 - leaked type "mitsuba.Object"
 - leaked type "mitsuba.Sensor"
 - leaked type "mitsuba.Integrator"
 - leaked type "mitsuba.ProjectiveCamera"
 - leaked type "mitsuba.Endpoint"
nanobind: leaked 61 functions!
 - leaked function "needs_aperture_sample"
 - leaked function ""
 - leaked function "get_medium"
 - leaked function "id"
 - leaked function "sampler"
 - leaked function "eval_direction"
 - leaked function ""
 - leaked function "pdf_direction"
 - leaked function "near_clip"
 - leaked function "__init__"
 - leaked function "__init__"
 - ... skipped remainder
nanobind: this is likely caused by a reference counting issue in the binding code.

These warnings do not occur when running a notebook containing the same code as the python script.

This seems similar if not identical to the issue at mitsuba-renderer/drjit#87, and this comment on that issue suggests that on shutdown of a script, some DrJit variables get freed in an undesired order, so it's not necessarily a problem, but I thought it worth adding this issue to track that open issue. In a notebook, the kernel continues running once the notebook has finished executing, which is why the warnings are not visible in notebooks.