[Feature] Add expectation value for density matrix

[EthanObadia]

Pyqtorch has recently incorporated noisy gates and circuit through the addition of Kraus operators #148.

Consequently, it becomes pertinent to modify the actual expectation() function to be able to support the noisy circuit, and thus the density matrix formalism. For a density matrix the expectation value of the observable $A$ is: $$\langle A \rangle = \text{Tr}(A \rho)$$

Also, Modify the sample method to support the DensityMatrix formalism