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

test/test_native_mha.py

@@ -213,7 +213,7 @@ class TestMHADeviceType(TestCase):
             q, k, v, key_padding_mask=mask if use_padding and not use_nt else None
         )
         if use_nt:
-            ynpt = ynpt.to_padded_tensor(0)
+            ynpt = torch.nested.to_padded_tensor(ynpt, 0)
             if pad_all:
                 ynpt_final = torch.zeros_like(ypt)
                 ynpt_final[:, :ynpt.shape[1], :] = ynpt