[SmokeTest] Do not import invalid_vec_isa

But rather coerce detected isa to bool, if it's false, that ISA is either invalid or unusable

test/smoke_test/smoke_test.py

@@ -260,9 +260,9 @@ def foo(x: torch.Tensor) -> torch.Tensor:
 
     # Check that SIMD were detected for the architecture
     if device == "cpu":
-        from torch._inductor.codecache import pick_vec_isa, invalid_vec_isa
+        from torch._inductor.codecache import pick_vec_isa
         isa = pick_vec_isa()
-        if isa == invalid_vec_isa:
+        if not isa:
             raise RuntimeError("Can't detect vectorized ISA for CPU")
         print(f"Picked CPU ISA {type(isa).__name__} bit width {isa.bit_width()}")