[fix] repr(torch.device) (#48655)

Summary:
Fixes #48585

In the following commit 4c9eb57, type of `DeviceIndex` was changed from `uint16_t` to `uint8_t`.
`uint8_t` is treated as ascii chars by std::cout and other stream operators. Hence the broken `repr`

Stackoverflow Reference: https://stackoverflow.com/questions/19562103/uint8-t-cant-be-printed-with-cout

Pull Request resolved: #48655

Reviewed By: bdhirsh

Differential Revision: D25272289

Pulled By: ezyang

fbshipit-source-id: a1549f5f8d417138cf38795e4c373e3a487d3691

test/test_torch.py

@@ -367,6 +367,17 @@ def test_device(self):
                 device_hash_set.add(hash(torch.device(device)))
             self.assertEqual(len(device_set), len(device_hash_set))
 
+            def get_expected_device_repr(device):
+                if device.index is not None:
+                    return "device(type='{type}', index={index})".format(
+                        type=device.type, index=device.index)
+
+                return "device(type='{type}')".format(type=device.type)
+
+            for device in device_set:
+                dev = torch.device(device)
+                self.assertEqual(repr(dev), get_expected_device_repr(dev))
+
         def test_to(self):
             def test_copy_behavior(t, non_blocking=False):
                 self.assertIs(t, t.to(t, non_blocking=non_blocking))

torch/csrc/Device.cpp

@@ -30,7 +30,10 @@ PyObject *THPDevice_repr(THPDevice *self)
   std::ostringstream oss;
   oss << "device(type=\'" << self->device.type() << "\'";
   if (self->device.has_index()) {
-    oss << ", index=" << self->device.index();
+    // `self->device.index()` returns uint8_t which is treated as ascii while printing,
+    // hence casting it to uint16_t.
+    // https://stackoverflow.com/questions/19562103/uint8-t-cant-be-printed-with-cout
+    oss << ", index=" << static_cast<uint16_t>(self->device.index());
   }
   oss << ")";
   return THPUtils_packString(oss.str().c_str());