Split up documentation into subpages and clean up some warnings (#37419)

Summary:
xref gh-32838, gh-34032

This is a major refactor of parts of the documentation to split it up using sphinx's `autosummary` feature which will build out `autofuction` and `autoclass` stub files and link to them. The end result is that the top module pages like torch.nn.rst and torch.rst are now more like table-of-contents to the actual single-class or single-function documentations pages.

Along the way, I modified many of the docstrings to eliminate sphinx warnings when building. I think the only thing I changed from a non-documentation perspective is to add names to `__all__` when adding them to `globals()` in `torch.__init__.py`

I do not know the CI system: are the documentation build artifacts available after the build, so reviewers can preview before merging?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37419

Differential Revision: D21337640

Pulled By: ezyang

fbshipit-source-id: d4ad198780c3ae7a96a9f22651e00ff2d31a0c0f

torch/_lowrank.py

@@ -84,7 +84,7 @@ def get_approximate_basis(A,        # type: Tensor
 
 def svd_lowrank(A, q=6, niter=2, M=None):
     # type: (Tensor, Optional[int], Optional[int], Optional[Tensor]) -> Tuple[Tensor, Tensor, Tensor]
-    """Return the singular value decomposition ``(U, S, V)`` of a matrix,
+    r"""Return the singular value decomposition ``(U, S, V)`` of a matrix,
     batches of matrices, or a sparse matrix :math:`A` such that
     :math:`A \approx U diag(S) V^T`. In case :math:`M` is given, then
     SVD is computed for the matrix :math:`A - M`.