Improve precision of emitted code for prim::Constant (#18817)
Summary:
Stacked on https://github.com/pytorch/pytorch/pull/18815 and https://github.com/pytorch/pytorch/pull/18811.
This makes it so that we emit a higher-precision literal for float values in the fusion kernel, as well as assign that to a `double` variable. This prevents us from losing precision for values such as `pi`, but with the previous fixes this will also get downcasted to `float` if downstream operations require it. Therefore, we should not lose performance because of implicit promotions
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18817
Differential Revision: D14820842
Pulled By: jamesr66a
fbshipit-source-id: 519671c6ca5e7adac746a4c4c72760a6d91e332f
