fix lint error

superbench/benchmarks/model_benchmarks/__init__.py

@@ -4,6 +4,7 @@
 """A module containing all the e2e model related benchmarks."""
 
 import sys
+from typing import Optional
 
 from superbench.benchmarks.model_benchmarks.model_base import ModelBenchmark
 from superbench.benchmarks.model_benchmarks.pytorch_bert import PytorchBERT
@@ -14,7 +15,7 @@
 from superbench.benchmarks.model_benchmarks.pytorch_llama import PytorchLlama
 
 # Check for Python version > 3.7 and conditionally import PytorchMixtral
-PytorchMixtral = None
+PytorchMixtral: Optional[type] = None
 if sys.version_info > (3, 7):
     from superbench.benchmarks.model_benchmarks.pytorch_mixtral import PytorchMixtral