32 replace tensornetwork simulator backend

[masato-fuk]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run pytest)
• format added code and tests by black -l 120 <filename>

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Context (if applicable): na

Description of the change: Replace TNbackend's dependent library from tensornetwork to quimb. File and class names are changed(mps.py -> tensornet.py, class MPS -> class TensorNetworkBackend). Most part of the basic usage remains unchanged.

Related issue:

32

also see that checks (github actions) pass. If lint check keeps failing, try installing black==22.8.0 as behavior seems to vary across versions.

[shinich1]

Also - it might be good to add tutorial explaining how the TN backend works (esp. 'opt' prep of graph state)? perhaps a topic for another issue though.

Could you also update examples and docs/source/simulator.rst

[masato-fuk]

@shinich1 I have completed all the changes for this issue, and I will make another issue to upgrade the examples and tutorial of the tensor network backend. Could you please review the changes?