Improvements to Comet Integration
Author: DN6
Pull request: yes

# What does this PR do?
I noticed that a lot of the `TrainingArguments` parameters were not being logged to [Comet](https://www.comet.ml/site/) with the latest version of  `transformers`. The issue seems to be in this line of the integration 

https://github.com/huggingface/transformers/blob/3772af49ceba348f2c9c5bbbb7f7c12e35d2a6eb/src/transformers/integrations.py#L614

The `args` variable containing the `TrainingArguments` parameters that the `CometCallback` uses is being overwritten with a dictionary. 

This PR: 
- fixes the issue with the parameter logging.
- adds the option of logging training assets (tf event logs, checkpoints etc) to Comet at the end of a run 
- Explicitly ends an Experiment at end of training (useful when running inside a Notebook)   

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?

## Regarding Testing

This affects the data being logged to Comet, so it has been tested on our side to ensure that the appropriate functionality is restored. I did not see any guidelines for testing integrations of this nature, but I am happy to include them as needed.   

## Who can review?
Tagging @sgugger since this is an integration related to `TrainerCallback` that affects the `Trainer` and `TrainerArguments` object. 

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