[Bug] dtype promotion to complex128 with tensorflow SProds _only_ in pauli_rep

[Qottmann]

We are currently promoting the dtype of tensorflow variables in the pauli rep of scalar products, this leads to some unexpected and inconsistent behavior, e.g.

op = qml.s_prod(tf.constant(0.5), X(0))
print(op.scalar)
print(next(iter(op.pauli_rep.values())))

(<tf.Tensor: shape=(), dtype=float32, numpy=0.5>,
 <tf.Tensor: shape=(), dtype=complex128, numpy=(0.5+0j)>)

This problem stems from this line https://github.com/PennyLaneAI/pennylane/blob/master/pennylane/ops/op_math/sprod.py#L152

[Alex-Preciado]

Closed with #5246