Fix a bug for `CallbackHandler.callback_list`
Author: harupy
Pull request: yes

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Fix a bug where `CallbackHandler.callback_list` fails when given callbacks are duplicated:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-40-9605b122f4d1> in <module>()
      2 from transformers.trainer import DEFAULT_CALLBACKS
      3 
----> 4 CallbackHandler(DEFAULT_CALLBACKS + [MLflowCallback], "model", "optimizer", "lr_scheduler")

2 frames
/usr/local/lib/python3.6/dist-packages/transformers/trainer_callback.py in __init__(self, callbacks, model, optimizer, lr_scheduler)
    277         self.callbacks = []
    278         for cb in callbacks:
--> 279             self.add_callback(cb)
    280         self.model = model
    281         self.optimizer = optimizer

/usr/local/lib/python3.6/dist-packages/transformers/trainer_callback.py in add_callback(self, callback)
    299                 f"You are adding a {cb_class} to the callbacks of this Trainer, but there is already one. The current"
    300                 + "list of callbacks is\n:"
--> 301                 + self.callback_list
    302             )
    303         self.callbacks.append(cb)

/usr/local/lib/python3.6/dist-packages/transformers/trainer_callback.py in callback_list(self)
    326     @property
    327     def callback_list(self):
--> 328         return "\n".join(self.callbacks)
    329 
    330     def on_init_end(self, args: TrainingArguments, state: TrainerState, control: TrainerControl):

TypeError: sequence item 0: expected str instance, DefaultFlowCallback found
```

Code to reproduce the bug:

```python
from transformers.trainer_callback import CallbackHandler
from transformers.trainer import DEFAULT_CALLBACKS

CallbackHandler(DEFAULT_CALLBACKS + [MLflowCallback], "model", "optimizer", "lr_scheduler")
```

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Comments:
Do I need to add a test in `tests/test_trainer_callback.py` verifying that instantiating a trainer with duplicated callbacks doesn't fail?

```python
# this should not fail
trainer = self.get_trainer(
    callbacks=[MyTestTrainerCallback, MyTestTrainerCallback],
)
```

@sgugger Thanks for the approval. I just added a test that verifies the following:

1. `Trainer` can be instantiated with duplicated callacks.
2. A warning is emitted for duplicated callbacks.