[rollout] feat: add support for discriminative reward model in reward loop

[yyDing1]

What does this PR do?

Reward computation in reward loop will follow the design below:

Reward Computation Logic:
- if user-customized reward function is provided:
    -> directly use user-customized reward function
- if user-customized reward function is not provided:
    -> rm is not enabled: use default rule-based reward function
    -> rm is disrm: compute reward score using disrm
    -> rm is genrm: raise error (user-costomized reward func must be provided)

This PR adds support for DisRM and polishes some code for GenRM and Rule-Based reward, with corresponding ci tests.

The legacy FSDP/Megatron reward models can soon be deprecated, as all scenarios are supported (i.e., rule, disrm, genrm).

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[wuxibin89]

we should also update document.