Megatron Inference does not support post-processed logprob return

**Describe the bug**

Once Megatron Inference gains a `logprobs-mode` argument, [71da32a9863780a7f85ca3084219ce66357b8925](https://github.com/NVIDIA-NeMo/RL/commit/71da32a9863780a7f85ca3084219ce66357b8925) must be reverted.

**Steps/Code to reproduce bug**

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A helpful guide on on how to craft a minimal bug report http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports.

**Expected behavior**

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**Additional context**

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