Introduce _contract_deltas

[LinjianMa]

"""
Given an input tensor network `tn`, remove redundent delta tensors
in `tn` and change inds accordingly to make the output `tn` represent
the same tensor network but with less delta tensors.
"""
function _contract_deltas(tn::ITensorNetwork)
  ...
end

"""
Given an input `partition`, contract redundent delta tensors in `partition`
without changing the tensor network value. `root` is the root of the
dfs_tree that defines the leaves.
Note: for each vertex `v` of `partition`, the number of non-delta tensors
  in `partition[v]` will not be changed.
Note: only delta tensors of non-leaf vertices will be contracted.
"""
function _contract_deltas(partition::DataGraph; root=1)
 ...
end

[codecov-commenter]

Codecov Report

Merging #73 (6e8391e) into main (e82baef) will increase coverage by 0.00%.
The diff coverage is 91.26%.

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@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   76.81%   76.82%           
=======================================
  Files          59       60    +1     
  Lines        3080     3115   +35     
=======================================
+ Hits         2366     2393   +27     
- Misses        714      722    +8     

Impacted Files	Coverage Δ
src/Graphs/abstractgraph.jl	57.89% <ø> (-2.11%)	⬇️
src/ITensorNetworks.jl	77.77% <ø> (ø)
src/contract_deltas.jl	87.50% <87.50%> (ø)
src/binary_tree_partition.jl	100.00% <100.00%> (+1.02%)	⬆️
src/itensors.jl	82.75% <100.00%> (+2.75%)	⬆️

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