Backend "ray_aws" cannot be used in regions other than us-east-1

The region parameter seems to be hardcoded in the RayConfigGenerator, which makes it impossible to use the Ray backend in regions other than `us-east-1`.

## To reproduce

Run the example from the tutorial in any region other than `us-east-1`.
```python
from autogluon.cloud import TabularCloudPredictor
import pandas as pd
train_data = pd.read_csv('https://autogluon.s3.amazonaws.com/datasets/Inc/train.csv')
predictor_init_args = {"label": "class"} # init args you would pass to AG TabularPredictor
predictor_fit_args = {"train_data": train_data, "time_limit": 120} # fit args you would pass to AG TabularPredictor
cloud_predictor = TabularCloudPredictor(
cloud_output_path=BUCKET, backend="ray_aws",
)
cloud_predictor.fit(
predictor_init_args=predictor_init_args,
predictor_fit_args=predictor_fit_args,
instance_type="ml.m5.2xlarge", # Check out supported instance and pricing here: https://aws.amazon.com/sagemaker/pricing/
wait=True, # Set this to False to make it an unblocking call and immediately return
)
```

This crashes because the login credentials are retrieved for `us-east-1` (hard-coded default) and the image is tried to be pulled from `eu-west-1` (current region).
```
2025-02-20 13:24:03,364 VINFO command_runner.py:371 -- Running `aws ecr get-login-password --region us-east-1 | docker login --username AWS --password-stdin 763104351884.dkr.ecr.us-east-1.amazonaws.com`
...
2025-02-20 13:24:06,605 VINFO command_runner.py:371 -- Running `docker pull 763104351884.dkr.ecr.eu-west-1.amazonaws.com/autogluon-training:1.2.0-cpu-py311`
Error response from daemon: Head "https://763104351884.dkr.ecr.eu-west-1.amazonaws.com/v2/autogluon-training/manifests/1.2.0-cpu-py311": no basic auth credentials
Shared connection to 34.247.195.30 closed.
2025-02-20 13:24:08,218 ERR updater.py:164 -- New status: update-failed
2025-02-20 13:24:08,218 ERR updater.py:166 -- !!!
2025-02-20 13:24:08,218 VERR updater.py:176 -- Exception details: {'show_color': None, 'message': 'SSH command failed.'}
2025-02-20 13:24:08,220 ERR updater.py:178 -- Full traceback: Traceback (most recent call last):
File "/local/home/shchuro/uv_envs/cloud/lib/python3.11/site-packages/ray/autoscaler/_private/updater.py", line 159, in run
self.do_update()
File "/local/home/shchuro/uv_envs/cloud/lib/python3.11/site-packages/ray/autoscaler/_private/updater.py", line 451, in do_update
self.cmd_runner.run_init(
File "/local/home/shchuro/uv_envs/cloud/lib/python3.11/site-packages/ray/autoscaler/_private/command_runner.py", line 722, in run_init
self.run(
File "/local/home/shchuro/uv_envs/cloud/lib/python3.11/site-packages/ray/autoscaler/_private/command_runner.py", line 493, in run
return self.ssh_command_runner.run(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/local/home/shchuro/uv_envs/cloud/lib/python3.11/site-packages/ray/autoscaler/_private/command_runner.py", line 379, in run
return self._run_helper(
^^^^^^^^^^^^^^^^^
File "/local/home/shchuro/uv_envs/cloud/lib/python3.11/site-packages/ray/autoscaler/_private/command_runner.py", line 298, in _run_helper
raise click.ClickException(fail_msg) from None
click.exceptions.ClickException: SSH command failed.
```