NNUE merge #2823
Description
The NNUE branch maintained by @nodchip has demonstrated strong results and offers great potential, and we will proceed to merge it into master. This will assure that Stockfish remains a reference engine based on the top computer chess technology for the foreseeable future. This merge will introduce machine learning based coding to the engine, thus enlarging the community of developers, bringing in new skills. We are eager to keep everybody on board, including all developers and users of diverse hardware, aiming to be an inclusive community. We will keep and evolve the handcrafted evaluation, not only for the beauty and insight it offers, but for additional reasons, including its value on diverse hardware platforms, and for the potential of hybridization with NNUE, and use in search. The initial goal for the merge is the NNUE playing capability only, this focus makes sense given the complexity of the project, and the aim of Stockfish as a production quality chess engine. We will try to make changes such that the @nodchip repo and ours can be kept in sync as easily as possible. Written by knowledgeable people, NNUE is already of good quality, we will make small modifications needed to make it pass our standard CI, and integrate nicely in the existing engine. Rigorous testing will continue to be a cornerstone of our development, and accordingly integration in fishtest needs to be properly implemented. Afterwards, we will guarantee continuous progression in playing strength testing core engine and networks with our usual SPRT process. Some initial steps of the merge will be outlined below, the precise steps needed will become clearer as we proceed, I look forward to working with the community to make this happen!