FPGAs are general purpose logic gates that you can reprogram on the fly to do whatever you want them to do. I suppose you could configure them to do matrix maths, but they are not going to rival a dedicated GPU. Most hardware that utilizes a FPGA does so to simplify circut design, allow for circut redesign after a board has been built, and to cut down the number of external logic parts needed.
A CPU / GPU also has many orders of magnitude more logic gates than an FPGA has.