Add single MeasurementValue support to measurement processes

[mudit2812]

Context:
PR 1 for adding support for collecting statistics for mid-circuit measurements. This PR adds support for single measurement values. Supported devices are default.qubit, default.mixed, and DefaultQubit2. Supported measurement processes are qml.expval, qml.var, qml.probs, qml.sample, qml.counts.

Description of the Change:

• Update QubitDevice expval, var, sample and statistics to handle measurement processes that have a MeasurementValue as their observable.
• Update qml.expval, qml.probs, qml.sample, qml.var, qml.counts to handle MeasurmentValues.
• Update ExpectationMP, VarianceMP, SampleMP, CountsMP to handle MeasurementValues as observables for state and sample processing.

measurements.rst will be updated in a follow-up PR.

Benefits:
Mid-circuit measurement statistics work.

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f2d784d) 99.62% compared to head (86fc10e) 99.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4544   +/-   ##
=======================================
  Coverage   99.62%   99.63%           
=======================================
  Files         375      375           
  Lines       33486    33536   +50     
=======================================
+ Hits        33362    33412   +50     
  Misses        124      124           

Files Changed	Coverage Δ
pennylane/measurements/__init__.py	100.00% <ø> (ø)
pennylane/_qubit_device.py	99.44% <100.00%> (+<0.01%)	⬆️
pennylane/devices/qubit/measure.py	100.00% <100.00%> (ø)
pennylane/measurements/counts.py	100.00% <100.00%> (ø)
pennylane/measurements/expval.py	100.00% <100.00%> (ø)
pennylane/measurements/measurements.py	100.00% <100.00%> (ø)
pennylane/measurements/mid_measure.py	100.00% <100.00%> (ø)
pennylane/measurements/probs.py	100.00% <100.00%> (ø)
pennylane/measurements/sample.py	100.00% <100.00%> (ø)
pennylane/measurements/var.py	100.00% <100.00%> (ø)
... and 1 more

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[timmysilv]

looking super good! I think there are just some remnant of the old implementation, but otherwise it's shaping up 👍

[timmysilv]

one last concern, but otherwise this looks ready to roll!

[timmysilv]

🎉 thanks for addressing all my questions! I am now realizing that the repr for a MeasurementProcess with a MeasurementValue (and the repr for those too) could use some work, but we can do it in another PR to get this one merged rn