Remove dependency on SciPy

[klane]

SciPy is only used for some sparse matrix operations in utils/graph.py. These include:

• scipy.sparse.coo_matrix
• scipy.sparse.csr_matrix
• scipy.sparse.diags
• scipy.sparse.eye
• scipy.sparse.identity (this just calls eye)
• scipy.sparse.issparse
• scipy.sparse.linalg.eigsh

If PyTorch has equivalent functionality in torch.sparse, then these should be ported over to PyTorch and SciPy should be removed as a dependency.