Source file for

I couldn’t find the source file for contained in L4T 32.3.1.
Where is it?

It is open source from r32.5.1. You may download the source code of r32.5.1 or r32.6.1 for reference.

There is a prebuilt lib for nvvidconv in r32.3.1:
Jetson/L4T/r32.3.x patches -
[GSTREAMER]Memory leak in video/x-raw ! nvvidconv ! video/x-raw(memory:NVMM)

The issue is specific. If you run a similar use-case, please apply it and give it a try.