Don't generate named tensor methods to {Tensor,TensorMethods}.h

[zou3519]

Stack from ghstack:

• Implement name inference for torch.dot #24474 Implement name inference for torch.dot
• Implement name inference for addmv, addmv_, mv #24471 Implement name inference for addmv, addmv_, mv
• Implement name inference for expand #24469 Implement name inference for expand
• Implement name inference for mm, addmm, addmm_ #24306 Implement name inference for mm, addmm, addmm_
• Fix binary op name inference between unnamed and named tensors. #24921 Fix binary op name inference between unnamed and named tensors.
• Add helper function Tensor::names() #24914 Add helper function Tensor::names()
• Rename Tensor::names() to Tensor::opt_names() #24907 Rename Tensor::names() to Tensor::opt_names()
• Fix FIXME_default_names by storing static list of 64 none names #24885 Fix FIXME_default_names by storing static list of 64 none names
• Add thread-local-state NamesMode and NoNamesGuard #24942 Add thread-local-state NamesMode and NoNamesGuard
• Implement name inference for t(), transpose(...) #24941 Implement name inference for t(), transpose(...)
• Don't generate named tensor methods to {Tensor,TensorMethods}.h #25022 Don't generate named tensor methods to {Tensor,TensorMethods}.h

Don't generate named tensor methods to {Tensor,TensorMethods}.h when
BUILD_NAMEDTENSOR=0 in an attempt to prevent fewer merge conflicts.

We can either fix the root cause by solving
#24931, or, in the short term,
go through with this (hacky) solution.

Here's what happens:

1. We filter out all named tensor declarations if BUILD_NAMEDTENSOR=0 so
   that they never hit the codegen.
2. What is checked in as {Tensor,TensorMethods}.h comes from a
   BUILD_NAMEDTENSOR=0 build.
3. When BUILD_NAMEDTENSOR=1, then GEN_TO_SOURCE is automatically set to
   1. When working on a named tensor build, one must remember not to
      check in the generated code.

Test Plan:

• run ci [namedtensor ci]

[gchanan]

let's hold off on this temporarily until we can figure out if we can just remove the src checkin.

[gchanan]

you should just change the default so it's true if BUILD_NAMEDTENSOR is on -- you should still be able to turn it off explicitly.