[AMD] Fix and enable upcasting fp8e4m3nv to fp16

[knwng]

This commit fixed and enabled fp8e4m3nv to fp16 upcasting.
In order to properly handle denorms with LUT, this operation is no longer vectorized.

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[antiagainst]

Can you also turn on the test by dropping these lines

https://github.com/triton-lang/triton/blob/3bac3be/python/test/unit/language/test_core.py#L3541-L3542