[seq2seq] fix logger format for non-main process
Currently, in `finetune_trainer.py` non-main process doesn't have any formatting at all, so we end up with:

```
[WARNING|modeling_t5.py:1645] 2021-01-30 20:01:37,246 >> [p0] got MPU
[WARNING|modeling_t5.py:1646] 2021-01-30 20:01:37,246 >> [p0] DP group [0]
[p1] got MPU
[p1] DP group [1]
```

as you can see the 2nd process in DDP misses formatting in logger.

this PR fixes it.

I looked in the take-over version `run_seq2seq.py` if it needed to be fixed too and it doesn't have these function calls, not sure why.  They appear to be needed, unless they get called elsewhere.

@sgugger, @patil-suraj 

@LysandreJik knows better for the centralized logging system so I'll defer to him.