[MLIR][Python] Add method for getting the live operation objects (#78663)

Currently, a method exists to get the count of the operation objects
which are still alive. This helps for sanity checking, but isn't
terribly useful for debugging. This new method returns the actual
operation objects which are still alive.

This allows Python code like the following:

```
    gc.collect()
    live_ops = ir.Context.current._get_live_operation_objects()
    for op in live_ops:
      print(f"Warning: {op} is still live. Referrers:")
      for referrer in gc.get_referrers(op)[0]:
        print(f"  {referrer}")
```