Extracting DCT coefficients from H.264/AVC bitstream
It is possible to extract the transform coefficients without fully decoding the entire picture, but you would still have to find your way through the bit stream, parsing whatever is necessary in order to arrive at every transform unit, which might also not be present for every coding unit (if some macroblock matches exactly one in a reference picture then you don't need to encode any residual, for example).
How you would use only the transform information to do your computer vision tasks, I have no idea, but it is entirely possible to only extract the transform coefficients. What i suggest you do is grab whatever standard you want (H.264 or H.265 or some other) and go through all of the tables with the syntax elements, and the parsing process for arriving at the transform coefficients would be to ignore every fixed width syntax element not useful for decoding the transform (anything related to motion vectors or reference pictures) and parse whatever is necesary to obtain all the transform units in the coded picture. I don't know if there is any tool out there that does this, perhaps you can go into ffmpeg or one of the libraries like x264 or x265 and modify them to extract what you want. It is a difficult task, but definitely possible.

Hope it helps