Add NNUE evaluation

[vondele]

This patch ports the efficiently updatable neural network (NNUE) evaluation to Stockfish.

Both the NNUE and the classical evaluations are available, and can be used to
assign a value to a position that is later used in alpha-beta (PVS) search to find the
best move. The classical evaluation computes this value as a function of various chess
concepts, handcrafted by experts, tested and tuned using fishtest. The NNUE evaluation
computes this value with a neural network based on basic inputs. The network is optimized
and trained on the evalutions of millions of positions at moderate search depth.

The NNUE evaluation was first introduced in shogi, and ported to Stockfish afterward.
It can be evaluated efficiently on CPUs, and exploits the fact that only parts
of the neural network need to be updated after a typical chess move.
The nodchip repository provides additional
tools to train and develop the NNUE networks.

This patch is the result of contributions of various authors, from various communities,
including: nodchip, ynasu87, yaneurao (initial port and NNUE authors), domschl, FireFather,
rqs, xXH4CKST3RXx, tttak, zz4032, joergoster, mstembera, nguyenpham, erbsenzaehler,
dorzechowski, and vondele.

This new evaluation needed various changes to fishtest and the corresponding infrastructure,
for which tomtor, ppigazzini, noobpwnftw, daylen, and vondele are gratefully acknowledged.

The first networks have been provided by gekkehenker and sergiovieri, with the latter
net (nn-97f742aaefcd.nnue) being the current default.

The evaluation function can be selected at run time with the Use NNUE (true/false) UCI option,
provided the EvalFile option points the the network file (depending on the GUI, with full path).

The performance of the NNUE evaluation relative to the classical evaluation depends somewhat on
the hardware, and is expected to improve quickly, but is currently on > 80 Elo on fishtest:

60000 @ 10+0.1 th 1
https://tests.stockfishchess.org/tests/view/5f28fe6ea5abc164f05e4c4c
ELO: 92.77 +-2.1 (95%) LOS: 100.0%
Total: 60000 W: 24193 L: 8543 D: 27264
Ptnml(0-2): 609, 3850, 9708, 10948, 4885

40000 @ 20+0.2 th 8
https://tests.stockfishchess.org/tests/view/5f290229a5abc164f05e4c58
ELO: 89.47 +-2.0 (95%) LOS: 100.0%
Total: 40000 W: 12756 L: 2677 D: 24567
Ptnml(0-2): 74, 1583, 8550, 7776, 2017

At the same time, the impact on the classical evaluation remains minimal, causing no significant
regression:

sprt @ 10+0.1 th 1
https://tests.stockfishchess.org/tests/view/5f2906a2a5abc164f05e4c5b
LLR: 2.94 (-2.94,2.94) {-6.00,-4.00}
Total: 34936 W: 6502 L: 6825 D: 21609
Ptnml(0-2): 571, 4082, 8434, 3861, 520

sprt @ 60+0.6 th 1
https://tests.stockfishchess.org/tests/view/5f2906cfa5abc164f05e4c5d
LLR: 2.93 (-2.94,2.94) {-6.00,-4.00}
Total: 10088 W: 1232 L: 1265 D: 7591
Ptnml(0-2): 49, 914, 3170, 843, 68

The needed networks can be found at https://tests.stockfishchess.org/nns
It is recommended to use the default one as indicated by the EvalFile UCI option.

Guidelines for testing new nets can be found at
https://github.com/glinscott/fishtest/wiki/Creating-my-first-test#nnue-net-tests

Integration has been discussed in various issues:
#2823
#2728

The integration branch will be closed after the merge:
#2825
https://github.com/official-stockfish/Stockfish/tree/nnue-player-wip

This will be an exciting time for computer chess, looking forward to seeing the evolution of
this approach.

Bench: 4746616

[MJZ1977]

Congratulation to all the team !
It is a big work. I am just impressed and impatient to make tests on these new features.

[Vizvezdenec]

I'm excited to retest my latest search patches for NNUE :)

[sf-x]

Any plans for SPSA optimization of pruning margins?

[Vizvezdenec]

I think we should make default for fishtest to use NNUE = true in tests.

[ddugovic]

If we intend to support both NNUE = false and NNUE = true, perhaps the test policy would be updated somehow to reflect this (I'm not suggesting the test bounds would need to stay the same for NNUE = false, esp. if resources are limited).

[vondele]

see also official-stockfish/fishtest#742 Use NNUE=true will be the default on fishtest, in particular for search patches. However, patches improving the classical eval will have to switch that default. This will become a little more tricky if/once evaluations hybridize. We will have to proceed using some common sense initially, which we afterwards turn into a better documented policy.

[MJZ1977]

I think that something easy without any drawbacks can be to show NNUE + classical evaluation in trace mode.

It can show difference between the 2 evaluations and then let us to make some hybrid combinations.

[vondele]

trace (eval command) will right now show the eval based on the state of UseNNUE.

[MJZ1977]

Yes, I have seen that. What I propose is something like that

• show classical evaluation in any case,
• if UseNNUE : also show NNUE evaluation.

[vondele]

merged!

Thanks @nodchip and everybody involved.

[vondele]

@Alayan-stk-2 please start an RT of current master (Use NNUE=true, auto-purge=off) at your convenience so we'll have a data point of the merge on the graphs.

[MichaelB7]

Well done by all. Not only only a huge plus for Stockfish - but a great win for computer chess. Thank you @nodchip for all of your contributions. @sergiovier , your nets are a thing of beauty, thank you. @vondele, you marshalled this commit masterfully, well done sir. Also, @FireFather, your contributions were pivotal in bringing this to fruition, thank you Norman.