Debug user callback transform custom plugin: does not trigger in Python behind `nvvideoconvert`

[aihsani]

UsercallbackTransform component does not trigger when inserted after nvvideoconvert.

• It works if inserted directly after nvstreammux; running in Python the custom plugin is triggered but
• It works after nvvideoconvert if used with gst-launch
  • Eg.

     gst-launch-1.0 uridecodebin uri=file:///app/videos/d1_im.mp4 ! mux.sink_0 nvstreammux name=mux width=1260 height=1024 batch-size=1 ! nvvideoconvert ! "video/x-raw(memory:NVMM),format=RGBA" ! usercallbacktransform ! nveglglessink sync=True
    

    works as expected. However, the same pipeline in Python bypasses usercallbacktransform completely and without errors