Move to using the Inference OpenAI API server

[ArEsKay3]

What does this PR do ?

This changes the RL loop to use the new inference server code that implements an OpenAI-esque API.
As a result we no longer need to worry about chat templating in the RL code (so all of that is removed).
Also, since the "non-chat" mode is a special case of the "chat" mode switch all RL code to use messages and add a fallback template that just concatenates requests.
In order to support output parsing it adds a new Parser class that takes a string and returns the remaining string and the processed metadata. Each inference library has their own versions of these, so we need our own. In the future hopefully we can rely on HF tokenizers like we do for chat templating.

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[ArEsKay3]

Two things I still need to clean up

• Countdown and Math environments want to control their prompt based on what stage of alignment the model is at (chat_mode). I think we should throw this away?
• We are not properly using the "n" argument to generation backend anymore. I want to move to use it where possible.

[yobibyte]

Looks so much cleaner now. Let me merge #2966 , and then I will merge main into this and work on nemogym support.

[yobibyte]

Will require adjusting of just recipes.

[tdene]

/ok to test 6baad1e

[ko3n1g]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/21932262559

[tdene]

/ok to test 14be3e1

[ko3n1g]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/21960186453

[ko3n1g]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/21962433107

[ko3n1g]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/21965383906