Fix applyMatrix in lightning.gpu

[multiphaseCFD]

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Context:

[SC-74899]

lightning.gpu stores gate data to a unordered_map object. The gate data can be accessed with a key (opsName, Param) . This design can avoid the excessive data copy between the device and host. However, the way a matrix gate is stored causes the bug since lightning.gpu will only add first matrix data to the cache since the keys of all matrix will be the same ({},{}) pair. Following applyMatrix calls will just result in getting data from the cache and the matrix used for operation will be always the first matrix. This bug will cause errors at least when applying TensorProd with multiple Hermitian obs.

To fix it, we will not store the matrix data to the gate cache and just copy it from host to device on-the-fly.

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.98%. Comparing base (133ab15) to head (27fbec0).
Report is 60 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #932      +/-   ##
==========================================
+ Coverage   96.41%   96.98%   +0.56%     
==========================================
  Files         215      132      -83     
  Lines       28165    17055   -11110     
==========================================
- Hits        27155    16540   -10615     
+ Misses       1010      515     -495     

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[LuisAlfredoNu]

LGTM, only a few questions.
Thank you @multiphaseCFD

[josephleekl]

Looks good to me! Thanks @multiphaseCFD!

[LuisAlfredoNu]

Thank you @multiphaseCFD
Nice work finding the 🐛