feat: refactor init of dtensor policy v2

[hemildesai]

Refactors init of dtensor policy v2 as part of #1589. Depends on #1695

Issues

#1589

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[github-actions]

⚠️ File Consistency Check

Check based on commit: 4f66b8f (PR #1709 from hemil/automodel-init-refactor)

⚠️ DTensor Policy Worker Synchronization Warning

The file nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py was modified in this PR, but nemo_rl/models/policy/workers/dtensor_policy_worker.py was not updated.

Why this matters:
These files contain related DTensor policy worker implementations that should be kept synchronized to ensure consistency across different versions.

Action required:

• Please review if the changes in nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py should also be applied to nemo_rl/models/policy/workers/dtensor_policy_worker.py
• Update nemo_rl/models/policy/workers/dtensor_policy_worker.py if necessary to maintain consistency
• If the files are intentionally different, please add a comment in the PR explaining why

Files to check:

• Modified: nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py
• Not modified: nemo_rl/models/policy/workers/dtensor_policy_worker.py

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_{This check ensures that related file implementations remain synchronized across the codebase. If you believe this warning is incorrect or the files should intentionally differ, please add a comment explaining the reasoning.}