perf(megatron-loss): scale logits per-chunk to avoid OOM

[Yangruipis]

# ⚡ Performance

## Move rollout_temperature division into per-chunk yield in get_responses

- Remove full-tensor `logits.div(rollout_temperature)` that allocated a
  duplicate `[T, V]` fp32 buffer (~16 GiB on Qwen3 with long packed
  sequences), doubling loss-step peak memory and triggering OOM under
  allocator fragmentation
- Apply the scalar division to each `logits_chunk` right before yielding,
  so allocations are bounded by per-sample response size and happen
  incrementally instead of as a single giant contiguous block
- Numerically equivalent across all four chunking paths (cp_size==1 RL,
  SFT, allgather_cp, zigzag CP) since scalar division commutes with
  slicing and concatenation