why is -hwaccel=output=format cuda not working?

[wonmanfactory]

I have read from nvidia transcoding guide saying that -hwaccel_output_format cuda option keeps the decoded frames in GPU memory.

i am trying to experimenting to convert a hevc_nvenc file to libx265 file to see the output file size by following command

ffmpeg -hwaccel cuda -hwaccel_output_format cuda -i .\test.mp4 -c:v libx265 test2.mp4

It just doesn't start the conversion. Prompting the following error

Impossible to convert between the formats supported by the filter 'Parsed_null_0' and the filter 'auto_scale_0'
Error reinitializing filters!
Failed to inject frame into filter network: Function not implemented
Error while processing the decoded data for stream #0:0

i thought it's only using GPU memory rather than computer memory. Why it will complain about format support?

If i remove the -hwaccel_output_format option, the conversion starts.

[wonmanfactory]

I just realised the reason
I get this error because the output is not nvenc encoder.