Fix the attribute name for torch device in argparser

[dongruoping]

Proposed change(s)

The argument dest="device" in add_argument() is using a different name than in

class TorchSettings:
    device: Optional[str] = parser.get_default("torch_device")

so it actually always gets None because the key ("torch_device") is not found, not because we set the default to None.

To trigger the issue, changing it to default=cuda and it will still get None by default.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

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