Add models playing chess example

docs/examples/models_playing_chess.md

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+# Large language models playing chess
+
+In this example we will make Mistral-7B play chess against a simple chess engine, ...
+
+## The chessboard
+
+The game will be played on a standard checkboard. We will use the `chess` [library](https://github.com/niklasf/python-chess) to track the opponents' moves, and check that the moves are valid. We will use the `chessboard` [library](https://pypi.org/project/chess-board/) to get a graphical representation of the current state of the game:
+
+```python
+import chess
+import chessboard.display
+
+board = chess.Board("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1")
+game_board = chessboard.display.start(board.fen())
+```
+
+## The opponents
+
+Mistral-7B quantized will be playing White:
+
+```python
+from outlines import models
+
+model = models.awq("model")
+```
+
+The X chess engine (3120 ELO) will be playing Black. We use the `chess` library to load the engine:
+
+```python
+engine = chess.engine.SimpleEngine.popen_uci(ENGINE_PATH)
+engine.configure({"UCI_LimitStrength": True,"UCI_Elo": 1320})
+```
+
+## A little help for the language model
+
+To make sure Mistral-7B generates valid chess moves we will use Outline's regex-guided generation. We define a function that takes the current state of the board and returns a regex that matches all possible legal moves:
+
+```python
+def generate_regex(board):
+    legal_moves = list(board.legal_moves)
+    move_strings = [board.san(move) for move in legal_moves]
+    # Remove + and # from the move strings
+    move_strings = [re.sub(r"[+#]", "", move) for move in move_strings]
+    regex_pattern = "|".join(re.escape(move) for move in move_strings)
+    regex_pattern = f"{regex_pattern}"
+    return regex_pattern
+```
+
+*Let's check how this works*
+
+## Prompting the language model
+
+The models' initial prompt
+
+```python
+prompt = "Score: 1-0 WhiteElo: 1600 BlackElo: 1600 Timecontrol: 1800+0 Moves: 1."
+```
+
+Update the prompt:
+
+```python
+def update_model_prompt(board, move):
+```
+
+## Let's play!
+
+
+```python
+while not board.is_game_over():
+    if board.turn == chess.WHITE:  # Mistral's turn
+        regex_pattern = generate_regex(board)
+        guided = generate.regex(model, regex_pattern, max_tokens=10)(prompt)
+        move = board.parse_san(guided)
+    else:  # Engine's turn
+        result = engine.play(board, chess.engine.Limit(time=0.6))
+        move = result.move
+
+    prompt = update_model_prompt(board, move)
+
+    board.push(move)
+    chessboard.display.update(board.fen(), game_board)
+```
+
+
+
+
+Idea: website with chess tournament.
+*This example was originally authored by [@903124S](@903124S) in [this gist](https://gist.github.com/903124/cfbefa24da95e2316e0d5e8ef8ed360d).*