Add initial LOBPCG top-k eigenvalue solver (#3112)

This initial version is f32-only for accelerators, since it relies on an eigh call (which itself is f32 at most) in its inner loop.

For details, see jax.experimental.linalg.standard_lobpcg documentation.

This is a partial implementation of the similar [scipy lobpcg
function](https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.linalg.lobpcg.html).

jax/experimental/sparse/__init__.py

@@ -259,3 +259,5 @@
     sparsify as sparsify,
     SparseTracer as SparseTracer,
 )
+
+from jax.experimental.sparse import linalg