[C10] Expand half unittest (#119892)
So far it's been only testing legacy conversion, rather than the one actually used when `at::Half` is constructed
Test `fp16` to `fp32` for the whole range of its 65536 values, though skip NaN comparisons, as different algorithms are not guaranteed to yield identical NaN representations and they are different anyway.
Do a small code cleanup, remove extraneous semicolons as well as named namespace inside unnamed one
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119892
Approved by: https://github.com/kit1980
