gptq: more obvious block var names

[vkuzo]

Summary:

block_start -> k_block_start
block_end -> k_block_end

This is the B variable from the paper.

Test Plan: CI

[vkuzo]

Stack from ghstack (oldest at bottom):

• gptq: more obvious inner loop var names #4227
• gptq: more obvious group var names #4226
• -> gptq: more obvious block var names #4225

[pytorch-bot]

🔗 Helpful Links

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As of commit 7ae41bc with merge base ce216da ():
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