Add index to HPU devices (#7497)

The [PR #7266](#7266)
enforces the devices having explicit device indices (i.e. 'hpu:0',
'cuda:0', etc).

This PR aligns HPU devices to the requirement.

Signed-off-by: Max Kovalenko <mkovalenko@habana.ai>

Author: deepcharm

accelerator/hpu_accelerator.py

@@ -54,8 +54,9 @@ def handles_memory_backpressure(self):
         return True
 
     def device_name(self, device_index=None):
-        # ignoring device_index.
-        return 'hpu'
+        if device_index is None:
+            return self._name
+        return '{}:{}'.format(self._name, device_index)
 
     def device(self, device_index=None):
         return torch.device(self.device_name(device_index))