Exponential operator wrapper

[albi3ro]

This wrapper allows us to represent the exponential of an operator. It has an optional keyword argument that allows the specification of a scalar coefficient. This allows us to easily represent general rotation gates.

In this example, we are able to mimic the behaviour of RX:

>>> x = np.array(1.23)
>>> op = Exp( qml.PauliX(0), -0.5j * x)
>>> qml.math.allclose(op.matrix(), qml.RX(x, wires=0).matrix())
True

The coefficient is optional and defaults to 1:

Exp(qml.PauliX(0))

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[codecov[bot]]

Codecov Report

Merging #2799 (da9aba0) into master (17a2317) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2799   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files         267      268    +1     
  Lines       22417    22489   +72     
=======================================
+ Hits        22340    22412   +72     
  Misses         77       77           

Impacted Files	Coverage Δ
pennylane/devices/default_qubit.py	100.00% <ø> (ø)
pennylane/operation.py	96.83% <ø> (ø)
pennylane/__init__.py	100.00% <100.00%> (ø)
pennylane/ops/op_math/__init__.py	100.00% <100.00%> (ø)
pennylane/ops/op_math/exp.py	100.00% <100.00%> (ø)

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