Arm backend: Fix mypy warnings in arm_model_eval

Fix mypy warnings in arm_model_evaluator.py

Change-Id: Ife2e5502d5dacd71e92bd5a86fdf25a34b0fd2b6

Author: perheld

backends/arm/util/arm_model_evaluator.py

@@ -59,7 +59,7 @@ def __init__(
         if tosa_output_path:
             self.tosa_output_path = tosa_output_path
         else:
-            self.tosa_output_path = None  # type: ignore[assignment]
+            self.tosa_output_path = ""
 
     def get_model_error(self) -> defaultdict:
         """
@@ -104,7 +104,7 @@ def get_compression_ratio(self) -> float:
 
         return compression_ratio
 
-    def evaluate(self) -> dict[Any]:  # type: ignore[type-arg]
+    def evaluate(self) -> dict[str, Any]:
         model_error_dict = self.get_model_error()
 
         output_metrics = {"name": self.model_name, "metrics": dict(model_error_dict)}