[DTensor] add __repr__ for CommDebugMode(get_total_count()=) (pytorch#165006)

I just want to print CommDebugMode and know if there is communication. implementing `__repr__` for `print(comm_mode)`

```
comm_mode = CommDebugMode()
with comm_mode:
    out = torch.mm(inps, weight)
print(comm_mode)
# CommDebugMode(get_total_counts()=0)
```

Tags:

Pull Request resolved: pytorch#165006
Approved by: https://github.com/anshul-si
ghstack dependencies: pytorch#165024

Author: weifengpy

torch/distributed/tensor/debug/_comm_mode.py

@@ -734,3 +734,6 @@ def __torch_dispatch__(self, func, types, args=(), kwargs=None):
         ].append(operation_dict)
 
         return out
+
+    def __repr__(self):
+        return f"CommDebugMode(get_total_counts()={self.get_total_counts()})"