Arm backend: Convert assert to throw ValueError in tosa_backend (#9905)

There's a risk with using asserts in production code as it might get
optimized out. A proper error is raised instead.

Signed-off-by: Sebastian Larsson <sebastian.larsson@arm.com>

Author: Sebastian-Larsson

Diff for: backends/arm/tosa_backend.py

@@ -75,12 +75,14 @@ def preprocess(  # noqa: C901
                 input_order = list(map(int, spec.value.decode().split(",")))
 
         # Check that the output format is set correctly in the compile spec
-        assert output_format == "tosa", "output format must be tosa"
+        if output_format != "tosa":
+            raise ValueError(f'Invalid output format {output_format}, must be "tosa"')
 
         tosa_spec = get_tosa_spec(compile_spec)
-        assert (
-            tosa_spec is not None
-        ), "TOSA backend needs a TOSA version specified in the CompileSpec!"
+        if tosa_spec is None:
+            raise ValueError(
+                "TOSA backend needs a TOSA version specified in the CompileSpec"
+            )
 
         logger.info(f"Converting ExportedProgram to TOSA: {tosa_spec}")