I tried this:

NVEncC64.exe -i "1.avs" -c raw -f raw --output-csp yuv420 --output-format yuv4mpegpipe -o - | ffplay -

But it seems one has to specify yuv420p, etc.

[yuv4mpegpipe @ 000001cb7edba800] ERROR: yuv4mpeg can only handle yuv444p, yuv422p, yuv420p, yuv411p and gray8 pixel formats. And using 'strict -1' also yuv444p9, yuv422p9, yuv420p9, yuv444p10, yuv422p10, yuv420p10, yuv444p12, yuv422p12, yuv420p12, yuv444p14, yuv422p14, yuv420p14, yuv444p16, yuv422p16, yuv420p16, yuva444p, gray9, gray10, gray12 and gray16 pixel formats. Use -pix_fmt to select one.
avout: failed to write header for output file: I/O error
failed to write output: unknown error..

How does one specify yuv420p or use -pix_fmt since that is an ffmpeg option?