[Bugfix][FP8] Fix dynamic FP8 Marlin quantization

[mgoin]

FIX #7216

We simply weren't expanding the per-tensor scales to channelwise for the fp16->fp8 dynamic quant case, which is needed for FP8 Marlin.

Also adds VLLM_TEST_FORCE_FP8_MARLIN environment variable so we can force testing of FP8 Marlin on GPUs with hardware support for FP8

[github-actions]

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[mgoin]

Hi @youkaichao I'm not sure the state of the build right now, but the failing tests seem to be unrelated. Lmk if you think I should investigate further

[youkaichao]

yes test failures are unrelated. I will hand it over to @robertgshaw2-neuralmagic for review on the quantization side.