[misc] feat: prototype deprecate DataProto and replace with Tensordict: part 3

[houminz]

What does this PR do?

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This PR continues the work started in PR #3567 by deprecating and removing the left_right padding mode

1. Implement no-padding mode for Megatron engine using nested tensors in sft trainer
2. Deprecating left_right padding mode for FSDP/Megatron engine
3. Introduces a transformation layer within Actor/Critic workers, see more here
   • Input Format:​​ Actor/Critic workers continue to receive data in left_rightpadded format.
   • ​​Transformation:​​ This layer dynamically converts left_rightpadded data into the no-padding format using nested tensors.
   • Engine Format:​​ FSDP and Megatron engines now operate exclusively using the no-padding data format by default.

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[gemini-code-assist]

Code Review

This pull request continues the work of replacing DataProto with Tensordict by deprecating and removing the left_right padding mode. The changes are consistently applied across dataset implementations, model engine logic, configurations, and tests, making no_padding the new default. The overall refactoring appears solid and well-executed. I've identified one issue: some debugging print statements were left in a test file and should be removed.