add personality to nanochat. breaks previous code on git pull and requires download of a new file from s3, but there is a helpful error message so hopefully its ok

tasks/customjson.py

@@ -0,0 +1,67 @@
+"""
+CustomJSON task for loading conversations from JSONL files.
+Each line in the JSONL file should be a JSON array of messages.
+"""
+
+import os
+import json
+from tasks.common import Task
+
+class CustomJSON(Task):
+    """
+    Load conversations from a JSONL file.
+    Each line should be a JSON array of message objects with 'role' and 'content' fields.
+    Example line: [{"role":"user","content":"Hi"},{"role":"assistant","content":"Hello"}]
+    """
+
+    def __init__(self, filepath, **kwargs):
+        super().__init__(**kwargs)
+        self.filepath = filepath
+        self.conversations = []
+
+        # Load all conversations from the JSONL file
+        if not os.path.exists(filepath):
+            # Helpful error message due to recent change. Will be removed in the future.
+            print("-" * 80)
+            print(f"Error: File {filepath} does not exist")
+            print("HINT (Oct 21 2025)")
+            print("If you recently did a git pull and suddely see this, it might be due to the new addition of identity conversations")
+            print("See this discussion for more details: https://github.com/karpathy/nanochat/discussions/139")
+            print("Quick fix: run the following command to download the file:")
+            print(f"curl -L -o {filepath} https://karpathy-public.s3.us-west-2.amazonaws.com/identity_conversations.jsonl")
+            print("-" * 80)
+            raise FileNotFoundError(f"File {filepath} does not exist")
+
+        with open(filepath, 'r') as f:
+            for line in f:
+                line = line.strip()
+                if not line:  # skip empty lines
+                    continue
+                messages = json.loads(line)
+
+                # Validate the conversation structure
+                assert isinstance(messages, list), f"Expected list of messages, got {type(messages)}"
+                assert len(messages) >= 2, f"Conversation must have at least 2 messages, got {len(messages)}"
+
+                # Validate message structure and alternating roles
+                for i, message in enumerate(messages):
+                    assert "role" in message, f"Message {i} missing 'role' field"
+                    assert "content" in message, f"Message {i} missing 'content' field"
+                    expected_role = "user" if i % 2 == 0 else "assistant"
+                    assert message["role"] == expected_role, f"Message {i} has role {message['role']} but should be {expected_role}"
+                    assert isinstance(message["content"], str), f"Message {i} content must be a string"
+
+                self.conversations.append(messages)
+
+        self.length = len(self.conversations)
+
+    def num_examples(self):
+        return self.length
+
+    def get_example(self, index):
+        messages = self.conversations[index]
+        conversation = {
+            "messages": messages,
+        }
+        return conversation
+