Remove abs from DefaultQubit

[dwierichs]

Context:
This PR implements the fix to #1125 proposed in this comment

Description of the Change:
The line 792

    prob = self.marginal_prob(self._abs(self._flatten(self._state)) ** 2, wires)

in qml/devices/default_qubit.py is replaced by

    flat_state = self._flatten(self._state)
    real_state = self._real(flat_state)
    imag_state = self._imag(flat_state)
    prob = self.marginal_prob(real_state ** 2 + imag_state ** 2), wires)

avoiding the usage of abs, which does not have a well-defined derivative at 0.

Benefits:
Fixes #1125 in all interfaces.

Possible Drawbacks:
Performance, see short discussion here

Related GitHub Issues:
#1125

[codecov]

Codecov Report

Merging #2057 (553e8c3) into master (20f1dd8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2057   +/-   ##
=======================================
  Coverage   99.17%   99.17%           
=======================================
  Files         225      225           
  Lines       17284    17291    +7     
=======================================
+ Hits        17142    17149    +7     
  Misses        142      142           

Impacted Files	Coverage Δ
pennylane/devices/default_qubit.py	100.00% <100.00%> (ø)
pennylane/devices/default_qubit_autograd.py	100.00% <100.00%> (ø)
pennylane/devices/default_qubit_jax.py	96.25% <100.00%> (+0.04%)	⬆️
pennylane/devices/default_qubit_tf.py	90.16% <100.00%> (+0.16%)	⬆️
pennylane/devices/default_qubit_torch.py	92.07% <100.00%> (+0.07%)	⬆️

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[josh146]

That was super quick, thanks @dwierichs! I agree, this is good to have fixed even with the minor overhead.

[josh146]

Minor, but we are in the process of slowly migrating the devices to qml.math :) Not an issue here though!

[dwierichs]

Thanks for the fast review! And very good idea to make the tests stable against possible default changes. :)