Better function annotations for `nn.functional` (#102918)
Fixes #102768
- Provides proper function declarations in generated `torch/nn/functional.pyi`.
- Moves some functions from manually defined in `functional.pyi.in` to generated code, in order to single-source the signature.
- Includes some of the functions in `torch._C._nn` into its `.pyi.in`, but not exhaustive (only what's already there).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102918
Approved by: https://github.com/drisspg, https://github.com/malfet
