symmetric backend very slow for PEPS tensors

[saeedjahromi]

Hey dev team. I have been working on a PEPS algorithms for simulating 2D fermionic systems with the symmetric backend and it turned out the symmetric backend is very slow for PEPS tensors. I did some profiling and found that the bottleneck is the slow performance of the functions in the blocksparse_utils.py file which mainly contributes to the _find_diagonal_sparse_blocks() and _find_transposed_diagonal_sparse_blocks() functions.

I was wondering if there is any solution to speed up the symmetric backend or any advice or suggestion from your side?