test(engine): add multi-sample token diversity validation test

Add test to verify multi-sample generation produces diverse tokens, validating broadcasting bug fix. Uses 10 samples with stochastic sampling to check 5-8 unique tokens are generated.

nanochat/engine.py

@@ -341,3 +341,37 @@ if __name__ == "__main__":
             print(f"Mismatch at {i}: {reference_ids[i]} != {generated_tokens[i]}")
             break
     print(f"Match: {reference_ids == generated_tokens}")
+
+    # Test multi-sample generation for token diversity
+    print("\n" + "=" * 60)
+    print("Testing token broadcasting fix...")
+    print("=" * 60)
+    
+    # Generate 10 samples with stochastic sampling
+    first_tokens = []
+    for token_column, token_masks in engine.generate(
+        prompt_tokens,
+        num_samples=10,
+        temperature=1.0,
+        top_k=50,
+        max_tokens=1,
+        seed=42
+    ):
+        # Extract first token from each sample
+        first_tokens = token_column
+        break  # Only need the first iteration
+    
+    # Calculate diversity metrics
+    unique_tokens = len(set(first_tokens))
+    
+    # Print results
+    print(f"Generated 10 samples")
+    print(f"First tokens: {first_tokens}")
+    print(f"Unique first tokens: {unique_tokens}/10")
+    
+    # Display pass/fail verdict
+    if unique_tokens > 1:
+        print(f"✅ PASSED: Multiple unique first tokens ({unique_tokens}/10)")
+        print("Note: With temperature=1.0, expect 5-8 unique tokens out of 10")
+    else:
+        print(f"❌ FAILED: All samples have the same first token (broadcasting bug still exists)")