Prevent unbounded growth of sparse tensor in add operation (#36030)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/34964
Sparse cuda add was implemented by just concatenating the indices and values for the tensor. If called repeatedly in a tight loop this will let `nnz` grow unbounded. In the worst case of `x.add_(x)` it grows exponentially.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36030
Differential Revision: D20873504
Pulled By: zou3519
fbshipit-source-id: d90ed8dda0c89571fb89e358757b5dde299513df
