As stated in the title I need to implement an auto-encoder on the LPC43S57JET256.
I would suggest asking the LPC43S57JET256 support question in the LPC Community which will be able to better assist you with that question.
For the CMSS-NN question, the CMSIS-DSP option must be selected in the MCUXpresso SDK builder and then you'll see the CMSIS-NN files. See my response to your other post.
Thanks for the reply. It's been a long time but we were now able to build the project using the CMSIS-NN-/DSP-library found on github(https://github.com/ARM-software/CMSIS_5) and LPCXpresso.