Relying on potentially uninitialized environment variables.

- https://github.com/aws/sagemaker-containers/blob/be2ac3746e0afeceb53d2f91261d1586ff290cfc/src/sagemaker_containers/_files.py#L156
- https://github.com/aws/sagemaker-containers/blob/0c0bfcd4afa54f2a7fd71ba6d5c814a692a636f0/src/sagemaker_containers/_trainer.py#L73

```
os.environ.get('AWS_REGION', os.environ.get(_params.REGION_NAME_ENV))
```
I don't think 'AWS_REGION' is set the platform in Training.
Looks like 'REGION_NAME_ENV' can be used independently but in reality it gets set when training environment is initialized.

Start at the referenced lines in src/sagemaker_containers/_files.py and src/sagemaker_containers/_trainer.py, then trace how the training environment initializes REGION_NAME_ENV. Check the behavior when AWS_REGION is absent and determine the expected independent use of REGION_NAME_ENV; done means the environment lookup no longer relies on a potentially unset variable.

Written by the indexing model from the issue text.