Fix a bug for CallbackHandler.callback_list

[harupy]

What does this PR do?

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:

from transformers.trainer_callback import CallbackHandler
from transformers.trainer import DEFAULT_CALLBACKS

CallbackHandler(DEFAULT_CALLBACKS + [MLflowCallback], "model", "optimizer", "lr_scheduler")

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to the it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@sgugger

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

[harupy]

Do I need to add a test in tests/test_trainer_callback.py verifying that instantiating a trainer with duplicated callbacks doesn't fail?

# this should not fail
trainer = self.get_trainer(
    callbacks=[MyTestTrainerCallback, MyTestTrainerCallback],
)

[sgugger]

Looking good, thanks for fixing! Any test you want to add is more than welcome as this new API is not super well tested yet.

[harupy]

@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.

[harupy]

The warning message emitted here looks like:

You are adding a <class 'tests.test_trainer_callback.MyTestTrainerCallback'> to
the callbacks of this Trainer, but there is already one. The currentlist of callbacks is
:DefaultFlowCallback
TensorBoardCallback
MyTestTrainerCallback

[harupy]

call_args[0][0] corresponds to the first positional argument of logger.warn

[sgugger]

Looks good!