[Bug]: FP8 checkpoints with fused linear modules fail to load scales correctly

[mgoin]

Your current environment

The output of `python collect_env.py`

🐛 Describe the bug

Description:
When loading FP8 quantized models with merged linear modules (e.g., Phi-3 with merged qkv_proj and up_gate_proj), the scales for each shard are not handled correctly. This occurs because the vLLM FP8 config assumes separate scales for each shard, but merged layers have a single scale.

Steps to Reproduce:

1. Attempt to load an FP8 quantized Phi-3 model (e.g., https://huggingface.co/nm-testing/Phi-3-mini-128k-instruct-FP8)
2. Observe error due to shape mismatch:

   param_data.shape=torch.Size([2]) loaded_weight.shape=torch.Size([])
   param_data.shape=torch.Size([3]) loaded_weight.shape=torch.Size([])
   

Expected Behavior:
Scales should be correctly loaded for merged linear modules in FP8 checkpoints.

Proposed Fix:
Modify process_weights_after_loading in MergedColumnParallelLinear and QKVParallelLinear to repeat the merged scale during weight loading.

Temporary Workaround:
Apply the following patch in vllm/model_executor/layers/linear.py:

- assert param_data.shape == loaded_weight.shape
- param_data.copy_(loaded_weight)
+ temp = loaded_weight.repeat(param_data.shape)
+ assert param_data.shape == temp.shape
+ param_data.copy_(temp)

cc @robertgshaw2-neuralmagic @comaniac

[comaniac]

I thought we handled this already? All the FP8 checkpoints have separated QKV scales and we merged them after weight loading. Is there anything special in Phi-3?

[robertgshaw2-neuralmagic]

Im working on a fix for this right now.

Issue is phi3 has fused qkv on disk so theres already only one scale!