[misc] feat: support offload parameter and optimizer during rollout

[PeterSH6]

• Fixed FSDP1 model offload
• With actor_rollout_ref.actor.fsdp_config.param_offload=True \ and actor_rollout_ref.actor.fsdp_config.optimizer_offload=True \ . The GPU memory utilization can increase to 0.9
• With actor, critic and reference offload all enabled, there will only be one model copy at a time in the GPU memory. Therefore, we can further increase the micro_batch_size_per_gpu or max_token_per_gpu

Specifically:

• During rollout, only rollout model and KVCache are in the GPU memory.
• During critic compute values, only the critic model will stay in the GPU memory while its optimizer and other model states are in CPU main memory
• During actor update, the actor model, optimizer are stored on GPU while the reference model and critic model, critic optimizer are offloaded to CPU.

[vermouth1992]

Shall we write a unit test for these two functions?

[PeterSH6]

Will add a CI in the future