Add `torch.nested` namespace (#84102)

First step towards #83775
- only `to_padded_tensor` is moved to the nested namespace for now
- following the schema used for `special`, `fft`, `linalg` and other namespaces, nested functions are registered in native_functions.yaml as `nested_{function_name}` and are bound to the desired Python name in
`torch/nested/__init__.py`, and the desired C++ name in `torch/csrc/api/include/torch/nested.h`.

~~**Question**: should we keep the documentation for `Tensor.to_padded_tensor` or can this deleted since it is shared by `torch.nested.to_padded_tensor`?~~

[generated nested docs](https://docs-preview.pytorch.org/84102/nested.html?highlight=nested#module-torch.nested)

Differential Revision: [D39361148](https://our.internmc.facebook.com/intern/diff/D39361148)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84102
Approved by: https://github.com/drisspg
diff --git a/tools/BUCK.bzl b/tools/BUCK.bzl
index 2ca59ee..e61ab02 100644
--- a/tools/BUCK.bzl
+++ b/tools/BUCK.bzl
@@ -129,6 +129,7 @@
             "autograd/templates/python_functions.cpp",
             "autograd/templates/python_functions.h",
             "autograd/templates/python_linalg_functions.cpp",
+            "autograd/templates/python_nested_functions.cpp",
             "autograd/templates/python_nn_functions.cpp",
             "autograd/templates/python_return_types.cpp",
             "autograd/templates/python_sparse_functions.cpp",