move import of VK 4-bit source quantizer into function

Differential Revision: D70268708

Pull Request resolved: #8744

Author: nathanaelsee

examples/models/llama/source_transformation/quantize.py

@@ -14,8 +14,6 @@
 import torch.nn as nn
 import torch.nn.functional as F
 
-from executorch.backends.vulkan._passes import VkInt4WeightOnlyQuantizer
-
 from executorch.extension.llm.export.builder import DType
 
 from sentencepiece import SentencePieceProcessor
@@ -180,6 +178,8 @@ def quantize(  # noqa C901
         model = gptq_quantizer.quantize(model, inputs)
         return model
     elif qmode == "vulkan_4w":
+        from executorch.backends.vulkan._passes import VkInt4WeightOnlyQuantizer
+
         q_group_size = 256 if group_size is None else group_size
         model = VkInt4WeightOnlyQuantizer(groupsize=q_group_size).quantize(model)