fix: wrap training loop in torch.enable_grad()

ComfyUI executes all nodes inside torch.no_grad(), which prevents gradient
tracking and makes loss.backward() fail. torch.enable_grad() overrides it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nodes/selva_lora_trainer.py

@@ -433,6 +433,8 @@ class SelvaLoraTrainer:
         print(f"\n[LoRA Trainer] Training {remaining} steps "
               f"(step {start_step + 1} → {steps})\n", flush=True)
 
+        # ComfyUI runs nodes inside torch.no_grad() — re-enable gradients for training
+        with torch.enable_grad():
             for step in range(start_step + 1, steps + 1):
                 x1_cpu, clip_f_cpu, sync_f_cpu, text_clip_cpu = random.choice(dataset)