[neurun] Revert to using const_cast for extendRank in Mul op (#4503)

[neurun] Revert to using const_cast for extendRank in Mul op (#4503)

* [neurun] Revert to using const_cast for extendRank in Mul op

This commit reverts to using `const_cast` for `extendRank` in `Mul` op.
That's because we've figured out the current implementation treats `copy` of `shape` object not `reference`.
It makes the `shape` not extend actually.

It seems it was a coincidence to be run all of tests related `broadcasting`.
(We need to check the tests...)

* add TrivialTest.BroadcastMulTwo test

Signed-off-by: sjsujinkim <sjsujin.kim@samsung.com>