nvfp4 gptq: prep for bmm and grouped_mm

[vkuzo]

Summary:

refactor to prepare for bmm/grouped_mm support

specifically:

1. make hessian init in the constructor to simplify
2. small fixes

Test Plan:

./torchao/prototype/gptq/gptq_nvfp4_llama3_2_1b_nonsequential_wikitext.sh
// accuracy unchanged

[vkuzo]

Stack from ghstack (oldest at bottom):

• [wip] perf improvements #4331
• make gptq convert work for moe #4330
• extend gptq example script with olmoe model #4329
• extend GPTQ coverage to grouped_mm #4328
• nvfp4 gptq for bmm #4327
• -> nvfp4 gptq: prep for bmm and grouped_mm #4323

[pytorch-bot]

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❌ 1 New Failure

As of commit 9efbf9f with merge base 899fea2 ():

NEW FAILURE - The following job has failed:

• Run 1xH100 Tests / test (H100, linux.aws.h100, --pre torch torchvision torchaudio mslk --index-url https://download.... / linux-job (gh)
  RuntimeError: Command docker exec -t 29ad1a03c8f83a1dbfbb169d4ff8cc97171d5508db586d5619ff52b7037c1c9c /exec failed with exit code 1

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