[MLA] Simplification to batch P/D reordering

[njhill]

I noticed that we're unnecessarily re-creating the sampling metadata twice when reordering the batch requests into prefill and decode groups for MLA.

This moves the reorder op from the start of the _prepare_inputs() method to the end of the _update_stats() method (which is called right before).

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[LucasWilkinson]

LGTM, do you mind just checking correctness?

VLLM_USE_V1=1 lm_eval --model vllm --model_args pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=2,dtype=auto,gpu_memory_utilization=0.9,trust_remote_code=True,max_model_len=16384,max_num_batched_tokens=1024 --task gsm8k --num_fewshot 5 

[njhill]

@LucasWilkinson eval results for this PR:

vllm (pretrained=deepseek-ai/DeepSeek-V2-Lite-Chat,tensor_parallel_size=2,dtype=auto,gpu_memory_utilization=0.9,trust_remote_code=True,max_model_len=16384,max_num_batched_tokens=1024), gen_kwargs: (None), limit: None, num_fewshot: 5, batch_size: 1
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.6672|±  |0.0130|
|     |       |strict-match    |     5|exact_match|↑  |0.6581|±  |0.0131|

Results from current main:

Chat,tensor_parallel_size=2,dtype=auto,gpu_memory_utilization=0.9,trust_remote_code=True,max_model_len=16384,max_num_batched_tokens=1024), gen_kwargs: (None), limit: None, num_fewshot: 5, batch_size: 1
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.6603|±  |0.0130|
|     |       |strict-match    |     5|exact_match|↑  |0.6535|±  |0.0131|

[LucasWilkinson]

LGTM, thanks for the accuracy checks!