Added Efficiency Improvements in the code. Including fast versions of…

[JoeyT1994]

This update adds efficiency improvements to the apply and gauge functions by minimising the amount of graph analysis that is done.

Additionally:

1. The apply function for a tn in the Vidal gauge places the site-index on the QR decomp relative to whether a gate is being applied.
2. A fast version of dag(tn::AbstractITensorNetwork) has been added
3. A function unflattened_norm_network(tn::AbstractITensorNetwork) has been added which is faster than using ⊗
4. An example has been added to demonstrate different gauging routines

[JoeyT1994]

Not really sure what is going on in test_apply here (which is why the tests are failing?). The changes to
function ITensors.apply( o::Union{ITensor,NamedEdge}, ψ::AbstractITensorNetwork, bond_tensors::DataGraph; normalize=false, apply_kwargs..., ) appear to have made the apply function extremely accurate for the simple update case and hence its accuracy is far higher than the belief propagation version (whereas they were identical before - as we expect them to be).

There must be a bug there as they should probably not be that accurate..

[mtfishman]

What if you switch back from qr(...) to factorize(...; cutoff=1e-16), do the tests pass? Otherwise it seems like the rest of the changes are either independent of those tests or just code reoarganization.

[mtfishman]

I'm confused by why you are saying SU gauging is now more accurate, it looks less accurate to me with the latest commit. EDIT: Never mind, I don't understand that test.

[JoeyT1994]

Okay those bugs should be fixed now and all tests should pass.

[codecov-commenter]

Codecov Report

Merging #94 (82b4909) into main (34fd5dc) will increase coverage by 0.10%.
The diff coverage is 90.90%.

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@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
+ Coverage   79.25%   79.36%   +0.10%     
==========================================
  Files          61       61              
  Lines        3442     3465      +23     
==========================================
+ Hits         2728     2750      +22     
- Misses        714      715       +1     

Impacted Files	Coverage Δ
src/apply.jl	89.26% <73.68%> (-1.96%)	⬇️
src/abstractitensornetwork.jl	82.66% <100.00%> (+0.77%)	⬆️
src/gauging.jl	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

[mtfishman]

Thanks!