Fix Comet ML integration
Author: mxschmdt
Pull request: yes



# What does this PR do?

This PR fixes an issue where the callback function `on_train_end` crashed if Comet ML integration was used but `experiment` was `None` after training (e.g. because the environment variable `COMET_MODE` was set to `DISABLE`).

Python snippet for testing (crashes before fix is applied):
```python
import os

from datasets import Dataset
from transformers import AutoModelForQuestionAnswering, AutoTokenizer, Trainer, TrainingArguments

# disable comet_ml logging
os.environ['COMET_MODE'] = 'DISABLE'

# create dummy dataset for training
tokenizer = AutoTokenizer.from_pretrained('bert-base-uncased')
inputs = tokenizer(["Who likes Pizza?"], ["Dave likes pizza"])
inputs['start_positions'] = [[6]]
inputs['end_positions'] = [[6]]
dataset = Dataset.from_dict(inputs)

# create trainer
trainer = Trainer(model=AutoModelForQuestionAnswering.from_pretrained('bert-base-uncased'), args=TrainingArguments(output_dir='tmp', max_steps=1), train_dataset=dataset)
trainer.train()
```

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## Who can review?

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Comments:
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