Lanczos Solver

[aamijar]

Lanczos Solver for Sparse Eigen Decomposition

We propose a new lanczos solver in raft that fixes the issues present in the previous solver raft::sparse::solver::detail::computeSmallestEigenvectors.

Specifically we address the following issues:

1. Numerical Stability for both float32 and float64 datatypes
2. Efficiency and Speed of Convergence

This new implementation is taken from the cupy library cupyx.scipy.sparse.linalg.eigsh where the thick-restart and full reorthogonalzation methods are used.

Additionally this PR exposes a python api for raft lanczos solver with an interface similar to scipy.sparse.linalg.eigsh and cupyx.scipy.sparse.linalg.eigsh.

from pylibraft.solver import eigsh

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[cjnolet]

I think this is coming along great, @aamijar. Mostly it needs some cleanup and polishing, but otherwise should be ready to merge once my comments are resolved.

[lowener]

Can a test for the python API of Lanczos also be added?

[lowener]

Can v0nrm and it's copy operations be skipped this way?

Suggested change

	[device_scalar = v0nrm_scalar.data_handle()] __device__(auto y) { return y / *device_scalar; });
	[device_scalar = output1.data_handle()] __device__(auto y) { return y / *device_scalar; });

[lowener]

I unfortunately don't have permissions to push on the branch of this PR so I kept the commits and continued on PR #2481.