Use fp16 for intermediate for fp8 weights with --fast if supported.

comfy/model_management.py

@@ -741,6 +741,9 @@ def unet_manual_cast(weight_dtype, inference_device, supported_dtypes=[torch.flo
         return None
 
     fp16_supported = should_use_fp16(inference_device, prioritize_performance=True)
+    if PRIORITIZE_FP16 and fp16_supported and torch.float16 in supported_dtypes:
+        return torch.float16
+
     for dt in supported_dtypes:
         if dt == torch.float16 and fp16_supported:
             return torch.float16