Cast initial state into complex dtype for default.qubit + Update Sum.label + fix qml.compile

[mudit2812]

• Upgrading qml to numpy 1.25 caused a bunch of ComplexWarnings to show up because we start out with a float initial state, which most likely gets cast into complex during execution. Then, during backprop, the complex state would get cast into float, which would raise the aforementioned warnings.

• Sum.label was also overly detailed, causing qml.draw to look terrible. I changed the label to be consistent with LinearCombination and Hamiltonian.

• qml.compile was not decomposing state prep ops, which is now changed.

[github-actions]

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[codecov]

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##             v0.38.0-rc0    #6198   +/-   ##
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  Misses                 ?      142           
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