Add missing conversion functions between half and float for ppc64le (#100168)
Fixes compilation error on ppc64-le resulting from missing conversion functions 'convert_half_float' and 'convert_float_half'.
These functions are implemented by this commit.
Started failing compilation from the following commit onwards: ced5c89b6fbe827a538b7ada96b2f9a5989871c7.
Fixes #ISSUE_NUMBER
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100168
Approved by: https://github.com/jgong5, https://github.com/ezyang
