Change AVX2/512 kernel infrastructure for additional gate/generator operations

[chaeyeunpark]

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---

Context: This PR extracts an essential structural change in #393. Especially, the SingleQubitGateHelper and TwoQubitGateHelper classes for AVX2/512 gates are clarified and tested.

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues:

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[codecov]

Codecov Report

Merging #404 (b3ea8bc) into master (f02becc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #404   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files          49       49           
  Lines        4463     4463           
=======================================
  Hits         4458     4458           
  Misses          5        5           

Impacted Files	Coverage Δ
pennylane_lightning/_version.py	100.00% <100.00%> (ø)
...gates/cpu_kernels/GateImplementationsAVXCommon.hpp	100.00% <100.00%> (ø)

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[AmintorDusko]

I left some comments for now. I'm still going through it, though.

[AmintorDusko]

More suggestions.

[multiphaseCFD]

Thanks @chaeyeunpark for the nice work! Just left a few comments.

[chaeyeunpark]

[sc-31399]

[mlxd]

Thanks a lot @chaeyeunpark
Mostly some minor doc edits and a few questions

[AmintorDusko]

Hey @chaeyeunpark, I have nothing more to add!
Nice work, here! Thanks!

[multiphaseCFD]

Nice work @chaeyeunpark !

[mlxd]

Thanks again @chaeyeunpark
Nothing new to add from me.

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