generate C++ API for meta functions using at::meta:: (#58570)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58570
**What the PR does**
Generate a fast-path `at::meta::{op}` API for calling meta functions without having to go through the dispatcher. This will be important for perf for external backends that want to use meta functions for shape checking (which seems likely to be what we end up doing for LazyTensorCore).
**Details**
In order to avoid naming collisions I had to make two small changes:
- rename `MetaFunctions.h` template -> `NativeMetaFunctions.h` (this is the file that declares the impl() function for every structured operator).
- rename the meta class: `at::meta::{op}::meta()` -> `at::meta::structured_{op}::meta()`
I also deleted a few unnecessary includes, since any file that includes NativeFunctions.h will automatically include NativeMetaFunctions.h.
**Why I made the change**
This change isn't actually immediately used anywhere; I already started writing it because I thought it would be useful for structured composite ops, but that isn't actually true (see [comment](https://github.com/pytorch/pytorch/pull/58266#issuecomment-843213147)). The change feels useful and unambiguous though so I think it's safe to add. I added explicit tests for C++ meta function calls just to ensure that I wrote it correctly - which is actually how I hit the internal linkage issue in the PR below this in the stack.
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D28711299
Pulled By: bdhirsh
fbshipit-source-id: d410d17358c2b406f0191398093f17308b3c6b9e
