Using distributed_backend='dp' causes result.write to fail
🐛 Bug
Saving results with result.write is broken up DP distributed training.
To Reproduce
Perform something like this
result.write('blah', ['one', 'two', 'three']
in a test_step and run with distributed_backend='dp', PTL will then call into Result.dp_reduce and try to run
self[k] = value.mean(dim=-1) where value is the predictions dictionary which will subsequently fail.