Fix exporting qnn quantized model (#3513)

Summary:
Pull Request resolved: #3513

As title, a small bug preventing qnn quantized model to be exported.

Reviewed By: cccclai

Differential Revision: D56954147

fbshipit-source-id: c8f900824ac9c7a94fdcb59c102c498e1c2efc3d

examples/models/llama2/export_llama_lib.py

@@ -374,7 +374,7 @@ def _export_llama(modelname, args) -> str:  # noqa: C901
     quantizers = get_pt2e_quantizers(pt2e_quant_params, args)
     quant_dtype = None
     if args.qnn and args.pt2e_quantize:
-        assert quantizers is None, "Should not enable both xnnpack and qnn"
+        assert len(quantizers) == 0, "Should not enable both xnnpack and qnn"
         qnn_quantizer, quant_dtype = get_qnn_quantizer(args)
         quantizers.append(qnn_quantizer)