new_contents: from tensornetwork.block_sparse import index
  from tensornetwork.block_sparse import charge
  from tensornetwork.block_sparse import blocksparsetensor
  from tensornetwork.block_sparse import linalg
  #pylint: disable=line-too-long
  from tensornetwork.block_sparse.blocksparsetensor import BlockSparseTensor, ChargeArray, tensordot, outerproduct
  #pylint: disable=line-too-long
  from tensornetwork.block_sparse.linalg import svd, qr, diag, sqrt, trace, inv, pinv, eye, zeros, ones, randn, eigh, eig, conj, reshape, transpose, random
  from tensornetwork.block_sparse.index import Index
  from tensornetwork.block_sparse.charge import U1Charge, BaseCharge
  
subject: add __init__.py (#503)