[external codegen] better yaml error messaging, added explicit error message tests (#56597)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56597
3 small changes, all centered around error messaging.
1) Improved error messages when `gen_backend_stubs.py` receives invalid yaml
2) Added error message tests. I wasn't sure if there was a canonical way to do this, so I just wrote a test that takes in a list of (yaml input, expected error message) pairs and runs the codegen pipeline on each of them.
3) I also removed the LineLoader from the yaml parsing bit that reads in the external backend yaml file. Two reasons that I took it out:
- The main reason we use it with native_functions.yaml is to easily pinpoint problems with new ops as they're added, that the codegen can pick up. 99% of these problems have to do with schema, which is irrelevant to the external yaml since it pulls the schema from native_functions
- Not all operators have to appear in the external yaml. We could do something like "line: -1", but that's kind of weird.
If you think the line numbers would actually be of more use than I'm thinking of in the external yaml though, let me know!
Test Plan: Imported from OSS
Reviewed By: navahgar
Differential Revision: D28474363
Pulled By: bdhirsh
fbshipit-source-id: 8b5ec804b388dbbc0350a20c053da657fad0474f
