[Bugfix] Fix empty (nullptr) channelwise scales when loading wNa16 using compressed tensors

[LucasWilkinson]

When running row parallel with channelwise scales, the integer divide in:

// in vllm/model_executor/layers/quantization/compressed_tensors/schemes/compressed_tensors_wNa16.py:
scales_and_zp_size = input_size_per_partition // group_size

would result in a 0 sized tensor (with shape (0, 8192), i.e. tensor([], device='cuda:2', size=(0, 8192), dtype=torch.bfloat16)) leading to a nullptr getting passed into marlin.

This is because when its channelwise group_size was getting set to input_size which is greater than input_size_per_partition in the row parallel case when tp > 1

Now for channelwise scales when running row parallel with a tp > 1 we replicate the scales to all gpus.

This issue was present for the model: "nm-testing/Meta-Llama-3.1-70B-Instruct-quantized.w8a16"

[github-actions]

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[tlrmchlsmth]

Thanks for tracking this down

[mgoin]

Nice debugging, thank you!