[XLA:Python] Use a string tag for devices in a heap profile rather th…

…an a numeric tag.

This makes filtering of heap profiles by device in pprof work reliably, e.g.:

```
pprof --tagfocus device=gpu:1 myprofile.prof
```

With a numeric filter, it was difficult to filter on device 0 (perhaps because proto3 conflates absent and default values for protobuf fields.)

PiperOrigin-RevId: 320998422
Change-Id: I537eba48a102f0f875aebf8452592d132e9947f1

tensorflow/compiler/xla/python/py_client.cc

@@ -275,7 +275,8 @@ py::bytes PyClient::HeapProfile() {
       kind_label->set_str(buffer_string_id);
       auto* device_label = sample->add_label();
       device_label->set_key(device_string_id);
-      device_label->set_num(entry.first.device->id());
+      device_label->set_str(
+          builder.StringId(entry.first.device->DebugString()));
     } else {
       kind_label->set_str(executable_string_id);
     }