[Dynamo] Do not crash if numpy is not installed (#112175)

`s/isinstance(value, np.generic)/np is not None and isinstance(value, np.generic)/`

Found while looking at #110512

Pull Request resolved: #112175
Approved by: https://github.com/ev-br, https://github.com/kit1980

torch/_dynamo/variables/builder.py

@@ -517,7 +517,7 @@ def index_source(key):
                 source=self.source,
                 guards=make_guards(GuardBuilder.ID_MATCH),
             )
-        elif isinstance(value, np.generic):
+        elif np is not None and isinstance(value, np.generic):
             # numpy array scalars: convert to 0D arrays
             return self.wrap_numpy_ndarray(np.asarray(value))
         elif is_numpy(value):