Fix Train LoRA crash when training_dtype is "none" with bfloat16 LoRA weights (#13145)

When training_dtype is set to "none" and the model's native dtype is
float16, GradScaler was unconditionally enabled. However, GradScaler
does not support bfloat16 gradients (only float16/float32), causing a
NotImplementedError when lora_dtype is "bf16" (the default).

Fix by only enabling GradScaler when LoRA parameters are not in
bfloat16, since bfloat16 has the same exponent range as float32 and
does not need gradient scaling to avoid underflow.

Fixes #13124

comfy_extras/nodes_train.py

@@ -1146,6 +1146,7 @@ class TrainLoraNode(io.ComfyNode):
         # Setup model and dtype
         mp = model.clone()
         use_grad_scaler = False
+        lora_dtype = node_helpers.string_to_torch_dtype(lora_dtype)
         if training_dtype != "none":
             dtype = node_helpers.string_to_torch_dtype(training_dtype)
             mp.set_model_compute_dtype(dtype)
@@ -1154,7 +1155,10 @@ class TrainLoraNode(io.ComfyNode):
             model_dtype = mp.model.get_dtype()
             if model_dtype == torch.float16:
                 dtype = torch.float16
-                use_grad_scaler = True
+                # GradScaler only supports float16 gradients, not bfloat16.
+                # Only enable it when lora params will also be in float16.
+                if lora_dtype != torch.bfloat16:
+                    use_grad_scaler = True
                 # Warn about fp16 accumulation instability during training
                 if PerformanceFeature.Fp16Accumulation in args.fast:
                     logging.warning(
@@ -1165,7 +1169,6 @@ class TrainLoraNode(io.ComfyNode):
             else:
                 # For fp8, bf16, or other dtypes, use bf16 autocast
                 dtype = torch.bfloat16
-        lora_dtype = node_helpers.string_to_torch_dtype(lora_dtype)
 
         # Prepare latents and compute counts
         latents_dtype = dtype if dtype not in (None,) else torch.bfloat16