Converting transformer_args to explicit object for checkpoint construction

[Jack-Khuu]

Side effect of 964d437 was that ModelArgs was updated to the following

    ModelArgs.transformer_args: Dict[str, Dict[str, Any]]

The checkpoint translation requires the field from TransformerArgs, so this creates an explicit object from the config

[pytorch-bot]

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❌ 2 New Failures

As of commit e6ed31a with merge base 3b162e2 ():

NEW FAILURES - The following jobs have failed:

• pull / test-mps / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / test-mps-dtype / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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