mx: add ceil and RNE rounding modes to the cast from fp32 to e8m0

[vkuzo]

Summary:

Why we want this: in the newly released cuBLAS 12.8 documentation, the
RNE rounding mode is used for the cast to e8m0. We want to properly
emulate this cast.

This is a copy-pasta of #516 with the
modifications being keeping FLOOR the default mode, and removing e3m0, credit
to @NicoleMayer for the original code. I don't have a way to check bitwise
equivalency with the most recent cuBLAS version yet, but will come back
and add tests when I do.

Test Plan:

pytest test/prototype/mx_formats/ -s -x

Reviewers:

Subscribers:

Tasks:

Tags:

[vkuzo]

Stack from ghstack (oldest at bottom):

• -> mx: add ceil and RNE rounding modes to the cast from fp32 to e8m0 #1620
• mx cleanup [2/x]: refactor mx gemm #1593

[pytorch-bot]

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