[QIM-2] Add a qml.math.state_to_density_matrix for matrices and qml.math.partial_trace to math module

[rmoyard]

Quantum Information module PR # 2
Context:

Currently the way to get the (reduced) density matrix from a matrix is a function in the device default.mixed.

Description of the Change:

Move the density matrix capabilities for state vector to the math module and implement partial_trace.
We add _density_matrix_from_matrix internal function that takes 2-d tensors for all interfaces as input and returns the (reduced) density matrix. It also add the matrix case to the function state_to_density_matrix which will be the user facing function that accepts state vectors, density matrices and QNodes returning states.

Benefits:

The reduced density matrix capabilities for matrices are now in the math module and it becomes a function accessible across the whole code base.

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

Merging #2569 (52d3740) into master (8332b0e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2569   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files         249      249           
  Lines       20307    20368   +61     
=======================================
+ Hits        20226    20288   +62     
+ Misses         81       80    -1     

Impacted Files	Coverage Δ
pennylane/devices/default_qubit.py	100.00% <ø> (ø)
pennylane/_qubit_device.py	99.09% <100.00%> (+0.30%)	⬆️
pennylane/devices/default_mixed.py	100.00% <100.00%> (ø)
pennylane/math/quantum.py	100.00% <100.00%> (ø)

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