ExpvalCost when optimize=True with shots batch

[rmoyard]

Context:
ExpvalCost returns wrong values when optimize=True because of the shape of results due to shots (e.g (8000,5)).

Description of the Change:
Change the function to take into account the shape of the results.

Benefits:
It now returns the correct values.

Related GitHub Issues:
Closes #1480

[github-actions]

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[codecov]

Codecov Report

Merging #1897 (457a917) into master (ec0b048) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1897   +/-   ##
=======================================
  Coverage   98.98%   98.98%           
=======================================
  Files         219      219           
  Lines       16555    16564    +9     
=======================================
+ Hits        16387    16396    +9     
  Misses        168      168           

Impacted Files	Coverage Δ
pennylane/vqe/vqe.py	100.00% <100.00%> (ø)

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[dime10]

Thanks @rmoyard, this looks good to me!