### What this PR does / why we need it?
Currently, when using pipeline parallel and pd disaggregate,
model_runner will return None on non-last-pp-rank stages in
`sample_tokens`, which will cause assert error in vllm
KVOutputAggregator on [this
line](https://github.com/vllm-project/vllm/blob/main/vllm/distributed/kv_transfer/kv_connector/utils.py#L84).
In fact, all pp workers should return a model_runner_output which
contains kv_connector_output to do aggregate in Enginecore scheduler
process to ensure all kv transfer is finished for kv cache releasing
later.
To fix this issue, this PR follows gpu_model_runner in vllm, passing
kv_connector_output in `sample_tokens` to make sure all ranks will
return a ModelRunnerOutput, in non-last-pp-rank workers, it will return
EMPTY_MODEL_RUNNER_OUTPUT with kv_connector_output.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
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Signed-off-by: lidenghui <lidenghui1110@gmail.com>
1c8c23de58