[feature] Backend options

[PabloAndresCQ]

Description

• Solves the issue below, allowing users to provide attributes from the cuTensorNet enums.
• Adds a check in sample from GeneralState to raise an exception if the circuit has no measurements.
• Small changes:
  • Default scratch_fraction is now 0.75
  • Set the default logging level of GeneralState to WARNING (less verbose than INFO)

Related issues

Solves #144

Checklist

• I have run the tests on a device with GPUs.
• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.

[jake-arkinstall]

lgtm!

Only snag is that the rename of the parameter attributes to cutn_attributes is a breaking change for any code that uses e.g. x.sample(n_shots=..., attributes=...). As long as this is acceptable then merge ahead.

[PabloAndresCQ]

Yeah, good point, about the breaking change. TBH it's not even that necessary, I did that because I intended to have cutn_attributes as an argument to the backends (and I just attributes felt too ambiguous), but that's not the case in the final approach I took. For GeneralState, leaving it as attributes is alright, so I'm going to roll it back.