Add typing.Result and typing.ResultBatch type variables

[albi3ro]

The results of an execution are difficult to concretely type hint due to the nested and dynamic shape of the object. Nonetheless, type hints are essential for clearly specific an interface and communicating what a function or method does.

pennylane.typing.Result, a typing.TypeVar specifies the result of a single execution. pennylane.typing.ResultBatch, a tuple of Result, specifies the result of a batch of executions.

I would prefer it if we could add documentation to a TypeVar, but at least this is an improvement and solves our current problem.

This PR already adds the type hint to a couple of key places, mostly for the new device interface. As time goes on, we can improve coverage of type hints.

[codecov]

Codecov Report

Merging #4108 (834a028) into master (5a36d6f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4108   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files         345      345           
  Lines       30920    30934   +14     
=======================================
+ Hits        30848    30862   +14     
  Misses         72       72           

Impacted Files	Coverage Δ
pennylane/devices/experimental/default_qubit_2.py	100.00% <100.00%> (ø)
pennylane/devices/experimental/device_api.py	98.00% <100.00%> (+0.12%)	⬆️
pennylane/devices/qubit/preprocess.py	100.00% <100.00%> (ø)
pennylane/devices/qubit/simulate.py	100.00% <100.00%> (ø)
pennylane/interfaces/execution.py	100.00% <100.00%> (ø)
pennylane/qnode.py	100.00% <100.00%> (ø)
pennylane/transforms/batch_transform.py	100.00% <100.00%> (ø)
pennylane/transforms/broadcast_expand.py	100.00% <100.00%> (ø)
pennylane/typing.py	100.00% <100.00%> (ø)

[rmoyard]

Thanks, it looks good! A couple of minor comments

[mudit2812]

Thanks Christina, this is coming along well. A few type hints need to be fixed, and I agree with @rmoyard 's suggestion about importing Result and ResultBatch from qml.typing. rather than using qml.typing.Result or qml.typing.ResultBatch.

[rmoyard]

Thanks!