Fix mapping of buffers in mmt4d module functions. (#10269)
Fix mapping of buffers in mmt4d module functions. (#10269)

When mapping the 4D buffers using the 2D buffer-mapping macro, I needed to multiply the 3 inner dims sizes together to form the coalesced inner dim of the 2D shape.

Rationalizing this also allowed to write a better explanatory comment: not longer had to make excuses for an "abuse" of the 2D macro.
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