[torchgen] Support multiple namespace in NativeFunctions.h (#79733)
Summary:
This is a follow up to #78015. This PR
* introduces namespace logic for generating `NativeFunctions.h`.
* adds helper function to extract namespace from string
* relaxes the constraint on the levels we support for custom kernel namespace to 2
Test Plan:
Yaml entry:
```
- func: unsqueeze.out(Tensor(a) self, int dim, *, Tensor(a!) out) -> Tensor(a!)
variants: function
device_check: NoCheck
dispatch:
CPU: custom_1::custom_2::unsqueeze
```
Generated `NativeFunctions.h`:
```
namespace custom_1 {
namespace custom_2 {
namespace native {
TORCH_API at::Tensor & unsqueeze(const at::Tensor & self, int64_t dim, at::Tensor & out);
} // namespace native
} // namespace custom_2
} // namespace custom_1
```
Differential Revision: D37198111
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79733
Approved by: https://github.com/bdhirsh
