Explicitly provide data types in default qubit device

[chaeyeunpark]

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Context: With ongoing effort to add SIMD/GPU supports in PennyLane-Lightning, we hope to have better support of a single-precision floating-point throughout the codebase. This PR deals with these requirements.

Description of the Change: QubitDevice, the parent class of all devices that support qubit operations, now accepts data types as an argument in the initializer. Some implicit type conversions happening in some methods became explicit to preserve data types.

Benefits: Offer more flexibility to devices to use their own data type.

Possible Drawbacks:

Related GitHub Issues:

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[codecov]

Codecov Report

Merging #2448 (c79f946) into master (5bad4fa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2448   +/-   ##
=======================================
  Coverage   99.53%   99.53%           
=======================================
  Files         243      243           
  Lines       19414    19436   +22     
=======================================
+ Hits        19323    19345   +22     
  Misses         91       91           

Impacted Files	Coverage Δ
pennylane/_device.py	98.06% <ø> (ø)
pennylane/_grad.py	100.00% <ø> (ø)
pennylane/gradients/parameter_shift_hessian.py	100.00% <ø> (ø)
pennylane/interfaces/autograd.py	100.00% <ø> (ø)
pennylane/interfaces/torch.py	100.00% <ø> (ø)
pennylane/numpy/tensor.py	97.53% <ø> (ø)
pennylane/ops/qubit/hamiltonian.py	100.00% <ø> (ø)
...nylane/transforms/optimization/pattern_matching.py	99.67% <ø> (ø)
pennylane/transforms/tape_expand.py	100.00% <ø> (ø)
pennylane/_qubit_device.py	98.76% <100.00%> (+0.03%)	⬆️
... and 16 more

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[antalszava]

Hi @chaeyeunpark, lots of apologies for the delayed review here 😿

The PR is looking great! 🙏

One definite change from my side would be to go from assert statements to errors. For the rest, I'm curious about your input too.

[chaeyeunpark]

Thanks for the detailed review @antalszava! I will incorporate the remaining points ASAP.

[antalszava]

Looks good 🥇 Awesome changes 🙂

[chaeyeunpark]

Thanks @antalszava for approving this.