Ok, I downloaded the source code of Leela Zero 0.17. Tell me, someone, in which file is the code that is described by this fragment of the article "Mastering Chess and Shogi by Self-Play with a General Reinforcement Learning Algorithm", p.13:

A move in chess may be described in two parts: selecting the piece to move, and then selecting among the legal moves for that piece. We represent the policy pi(a|s) by a 8x8x73 stack of planes encoding a probability distribution over 4,672 possible moves. Each of the 8x8 positions identifies the square from which to “pick up” a piece. The first 56 planes encode possible ‘queen moves’ for any piece: a number of squares [1::7] in which the piece will be moved, along one of eight relative compass directions fN;NE;E; SE; S; SW;W;NWg. The next 8 planes encode possible knight moves for that piece. The final 9 planes encode possible underpromotions for pawn moves or captures in two possible diagonals, to knight, bishop or rook respectively. Other pawn moves or captures from the seventh rank are promoted to a queen.

Chess
Feature Planes
Queen moves 56
Knight moves 8
Underpromotions 9
Total 73
Table S2: Action representation used by AlphaZero in Chess and Shogi respectively. The policy is represented by a stack of planes encoding a probability distribution over legal moves; planes correspond to the entries in the table.