Add basic ldexp operator for numpy compatibility (#45370)
Summary:
Adds ldexp operator for https://github.com/pytorch/pytorch/issues/38349
I'm not entirely sure the changes to `NamedRegistrations.cpp` were needed but I saw other operators in there so I added it.
Normally the ldexp operator is used along with the frexp to construct and deconstruct floating point values. This is useful for performing operations on either the mantissa and exponent portions of floating point values.
Sleef, std math.h, and cuda support both ldexp and frexp but not for all data types. I wasn't able to figure out how to get the iterators to play nicely with a vectorized kernel so I have left this with just the normal CPU kernel for now.
This is the first operator I'm adding so please review with an eye for errors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45370
Reviewed By: mruberry
Differential Revision: D24333516
Pulled By: ranman
fbshipit-source-id: 2df78088f00aa9789aae1124eda399771e120d3f
