fix: update device handling in OpusMtEnESTransformer

[Irozuku]

Improve Device Selection in OpusMtEnESTransformerSchema

This pull request updates the OpusMtEnESTransformerSchema in opus_mt_en_es_transformer.py to dynamically detect and enumerate available hardware devices (CPU and CUDA GPUs) for model training. This makes the device selection more robust and reflective of the actual environment, improving usability and reducing configuration errors.

Device Selection Improvements

• Added dynamic detection of available CUDA devices using torch, and updated the DEVICE_ENUM and DEVICE_PLACEHOLDER variables to reflect the actual hardware present.
• Updated the device field in the OpusMtEnESTransformerSchema to use the dynamically determined DEVICE_ENUM and DEVICE_PLACEHOLDER instead of hardcoded values.

Before (without cuda)

After (without cuda)