torch.btrifact for tensors with greater than 3 dimensions (#14964)

Summary:
Motivation:
- Earlier, `torch.btrifact` could not handle tensors with greater than 3 dimensions. This is because of the check:
>   AT_CHECK(THTensor_(nDimension)(a) == 3, "expected 3D tensor, got size: ", a->sizes());

What is in this PR?:
- Move `btrifact` to ATen
- Remove relation to TH/THC.
- Handle tensors with more than three dimensions
- Tests
- Docs modifications: added a note about the non-pivoting variant.

[blocked due to old magma-cuda binaries]
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14964

Differential Revision: D14405106

Pulled By: soumith

fbshipit-source-id: f051f5d6aaa45f85836a2867176c065733563184

torch/_torch_docs.py

@@ -5411,6 +5411,11 @@ Batch LU factorization.
 Returns a tuple containing the LU factorization and pivots. Pivoting is done if
 :attr:`pivot` is set.
 
+.. note::
+    LU factorization with :attr:`pivot` = ``True`` is not available for CPU, and attempting
+    to do so will throw an error. However, LU factorization with :attr:`pivot` = ``True`` is
+    available for CUDA.
+
 Arguments:
     A (Tensor): the tensor to factor
     pivot (bool, optional): controls whether pivoting is done