[Model] Support quantization of PixtralHFTransformer for PixtralHF

[mgoin]

Allows us to support quantization of the vision encoder for PixtralHF, where before we could only quantize the language model. Also converts the raw torch nn.Linear modules to proper merged vLLM Linear modules

Tested with

• neuralmagic/pixtral-12b-FP8-dynamic (only MistralModel quantized)
• nm-testing/pixtral-12b-FP8-dynamic-all (MistralModel and PixtralHFTransformer quantized, just not lm_head)

[github-actions]

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[ywang96]

LGTM! Not related to this PR in particular but does it make sense to use get_act_and_mul_fn and act_and_mul everywhere?

[mgoin]

Yeah this was my thinking @ywang96, with get_act_and_mul_fn now we should be able to replace most hardcoded SiluAndMul usages. First I'm going to look to see if we need this ScaledActivation case anymore