Parallel hamiltonian

[minhtriet]

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• [x] All new features must include a unit test. If you've fixed a bug or added code that should be tested, add a test to the test directory!

• [x] All new functions and code must be clearly commented and documented. If you do make documentation changes, make sure that the docs build and render correctly by running make docs.

• [x] Ensure that the test suite passes, by running make test.

• [x] Add a new entry to the doc/releases/changelog-dev.md file, summarizing the change, and including a link back to the PR.

• [x] The PennyLane source code conforms to PEP8 standards. We check all of our code against Pylint. To lint modified files, simply pip install pylint, and then run pylint pennylane/path/to/file.py.

When all the above are checked, delete everything above the dashed line and fill in the pull request template.

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