Stochastic lanczos quadrature

[devansh20la]

Changes

1. Added stochastic_lanczos.py under hessian_eigenthings. As per the discussions on the OG repo, the hvp operations are performed on gpu while the rest of the computations are performed on the CPU with float64 precision.
2. tests/spectral_density_tests.py. As of now I only perform statistical distance test here. I wanted to make this pull request and get the conversation going.
3. General housekeeping in "__init__.py"

• Add support for multi-GPU.