Reorganize order of operations in inference context and text generation controller

[tdene]

What does this PR do ?

This PR cleans up the order of operations in order to easily enable future optimizations.

In addition, this PR adds some common sugar that is used by multiple such optimizations.

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[Phlip79]

Please add tests for the new logic.

[santhnm2]

Can we consolidate this with the last_token_logits function in the context? I suspect either this code or that code will be redundant now but I'm not sure what the exact implications are

[tdene]

I believe this is done.

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/25137188347