Fix GPTQWeight import (#2020)

# What does this PR do?

Fix stray import.

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server/text_generation_server/layers/gptq/exllama.py

@@ -1,4 +1,4 @@
-from text_generation_server.utils.weights import GPTQWeight
+from text_generation_server.layers.gptq import GPTQWeight
 import torch
 from exllama_kernels import make_q4, q4_matmul, prepare_buffers, set_tuning_params