Report Approximate Truncation Error from Apply Function

[JoeyT1994]

This small PR allows passing a truncation_error! reference to the apply function in the case of the simple update routine. This will be filled in with the truncerr from the corresponding call to factorize_svd allowing one to keep track of the approximate error induced by the simple update routine.

Note that this is not possible to infer solely from the singular_values! reference object due to the fact that the spectrum is not normalized by factorize_svd!.

Tests are updated in apply_test.jl to make sure the functionality is working.

[mtfishman]

@JoeyT1994 take a look at the callback design in ITensorMPS.truncate!: https://github.com/ITensor/ITensorMPS.jl/blob/v0.3.17/src/abstractmps.jl#L1692-L1714 (introduced in ITensor/ITensorMPS.jl#99), I think that could be a good code pattern to use here as well since it is more general (it makes it easier for users to decide how to store the output, output different combinations of values, etc.).

[JoeyT1994]

@mtfishman Oh yeah that's quite a nice way to do it. I have updated the code to use a callback for the ITensorNetworks interface instead

[JoeyT1994]

@mtfishman I can't say I understand the reason for the failing tests. I am not seeing anything locally, it seems to be erroring here when running test_contraction_sequence_to_graph.jl but not reporting the reason for the exit code.

[mtfishman]

Looks good, thanks! I have no idea what's going on with those tests, I've seen that issue randomly but only in this package. Maybe one of the tests uses a lot of memory?

[JoeyT1994]

Yeah maybe next time we see it I can try to remove the test that causes it