[sglang] fix: remove unused padding in SGLang rollout

[PopSoda2002]

What does this PR do?

What does this PR do?
There are some unused padding talked in this issue:
zhaochenyang20/Awesome-ML-SYS-Tutorial#193

• There are just 5 key fields which need to return back after rollout(example in agent_loop):

batch = TensorDict(
{
    "prompts": prompt_ids,  # [bsz, prompt_length]
    "responses": response_ids,  # [bsz, response_length]
    "response_mask": response_mask,  # [bsz, response_length]
    "input_ids": input_ids,  # [bsz, prompt_length + response_length]
    "attention_mask": attention_mask,  # [bsz, prompt_length + response_length]
    "position_ids": position_ids, 
    # position_ids: [bsz, 3, prompt_length + response_length] or [bsz, prompt_length + response_length]
},
batch_size=len(inputs),
)

• Remove some unused variable like prompt_loss_mask
• Make response_position_id all zero tensor
• Copy class to avoid constructing a new class

Test

over_sample = 0.1
wandb

No issue.

over_sample = 0.0
wandb

As expected too

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[zhaochenyang20]

Good to go. pass the CI plz