Deprecate the internal switching of a backpropagation device

[albi3ro]

Context:

The internal switching of one device for another backpropagation-compatible device was developed to allow the substitution of default.qubit.legacy with either default.qubit.autograd, default.qubit.jax, default.qubit.torch, or default.qubit.tf. This logic has caused a lot of pain over the years, and all five of those devices are now deprecated. In order to complete the removal of the 5 legacy devices, we need to also deprecate and remove the backprop device switching logic.

Otherwise, it would be very difficult and painful to test the switching of a backpropagation device.

Description of the Change:

Deprecates the switching of a device with a different device. A warning is only raised when the device is not default.qubit.legacy, as default.qubit.legacy raises it's own deprecation warnings.

Benefits:

We can finally clean up QNode 🎉 💥

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (b7675ca) to head (8d48d9e).
Report is 299 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6032      +/-   ##
==========================================
- Coverage   99.66%   99.65%   -0.01%     
==========================================
  Files         428      428              
  Lines       41182    40889     -293     
==========================================
- Hits        41042    40748     -294     
- Misses        140      141       +1     

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