Update portable op names to match functions.yaml (#1049)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1049

- selected_op.yaml; contains *op_names* collected from the model
- dtype selective build; for each *op_name*, generate kernel code only if the dtype is used in the model

- this diff: make sure op_names in op_<op>.cpp file match the op name collected by selected_op.yaml. This is the op name from [functions.yaml](https://fburl.com/code/uok7njmb)

Note: the op names are not used for anything besides selective build, so this change should not break anything.

Reviewed By: manuelcandales

Differential Revision: D50425923

fbshipit-source-id: e91932d67be3bad1369f6065a02836abdd7490fd