In MlflowCallback, Can not reattach to an existing
# In Documentation
[Doc](https://huggingface.co/docs/transformers/main_classes/callback#transformers.integrations.MLflowCallback) said we can reattach to an existing run by set **MLFLOW_RUN_ID** environment variable.

# In Code
But [Code](https://github.com/huggingface/transformers/blob/118e9810687dd713b6be07af79e80eeb1d916908/src/transformers/integrations.py#L990) seems making new mlflow run with **args.run_name**.

# ASIS
- code at https://github.com/huggingface/transformers/blob/118e9810687dd713b6be07af79e80eeb1d916908/src/transformers/integrations.py#L990
```
self._ml_flow.start_run(run_name=args.run_name, nested=self._nested_run)
```

# TOBE
```
self._ml_flow.start_run(run_id=self._run_id, nested=self._nested_run)
```
Hi @ykihong0, thanks for reporting this issue. 

The integrations are community added and maintained by their authors - Pinging @orieg who I believe added the env var logic in #17130 :)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.