Stop checking in VmapGeneratedPlumbing.h (#82351)

This PR changes VmapGeneratedPlumbing.h to be generated by torchgen. The
output file is ATen/VmapGeneratedPlumbing.h.

Why generate this file inside PyTorch codegen instead of a separate step
in functorch?
- I can't figure out how to get functorch's fbcode target to generate
- functorch's build system will, in the mid-term, be absorbed into
pytorch's build system, so I don't want to do the extra work of adding
a step to the functorch build process.

Test Plan:
- build pytorch, build functorch
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82351
Approved by: https://github.com/ezyang

build.bzl

@@ -144,6 +144,7 @@ GENERATED_H = [
     "FunctionalInverses.h",
     "RedispatchFunctions.h",
     "RegistrationDeclarations.h",
+    "VmapGeneratedPlumbing.h",
 ]
 
 GENERATED_H_CORE = [