[rollout] fix: some compatibility changes in agent loop and reward

[pengwu22]

What does this PR do?

Some compatibility changes, including

• agent_loop:
  • compatible with model without system prompt
  • compatible with other multi-modal model with processor available
• reward:
  • allow override_config for huggingface model

Test

• train Qwen VL and other internal multi-modal models with customized reward on agent loop
• CI

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