fix(milestone): reset model before mixed task challenge

The transformer was being trained sequentially on reversal then copying,
which caused it to "forget" reversal before the mixed task. Now we
reinitialize the model before challenge 3 so it learns both tasks
together with proper prefix conditioning.

tinytorch/milestones/05_2017_transformer/01_vaswani_attention.py

@@ -544,6 +544,20 @@ def main():
     results['copying'] = (passed2, acc2)
 
     # Challenge 3: Mixed Tasks (the real test)
+    # Reset model and optimizer so it learns both tasks from scratch
+    # with prefix conditioning (otherwise it's stuck in "copy mode")
+    console.print("\n[dim]Reinitializing model for mixed task learning...[/dim]")
+    model = AttentionTransformer(
+        vocab_size=vocab_size,
+        embed_dim=embed_dim,
+        num_heads=num_heads,
+        seq_len=seq_len + 1,
+        num_layers=num_layers
+    )
+    for param in model.parameters():
+        param.requires_grad = True
+    optimizer = Adam(model.parameters(), lr=lr)
+
     train_mixed = generate_mixed_data(800, seq_len)
     test_mixed = generate_mixed_data(300, seq_len)
     passed3, acc3 = run_challenge(