Better assertion message in vectorizing_map.

One situation that triggers this assertion is when trying to tf.print(pfor_config.collect(something)).  Arguably that should just work, but failing that, having the error indicate the nature of the problem is helpful.

PiperOrigin-RevId: 249108396

tensorflow/python/ops/parallel_for/pfor.py

@@ -993,7 +993,8 @@ class PForConfig(object):
 
   def _lookup_reduction(self, pl):
     """Lookups Placeholder `pl` in the reduction map."""
-    assert isinstance(pl, ops.Tensor)
+    msg = "Expected Tensor, got {} of type {}."
+    assert isinstance(pl, ops.Tensor), msg.format(pl, type(pl))
     return self._reduce_concat_map.get(pl, None)