[WIP] Parallel batch execution for VQC

[BrunoLiegiBastonLiegi]

This implements the parallel execution of batches of different initial states for the same initial circuit. This is particularly helpful for amplitude encoding based VQCs @Edoardo-Pedicillo.

What is supported:

• getting the batch of final states
• getting the batch of final samples
• getting the batch of final probabilities

What is not (yet) supported or tested:

• getting the final frequencies
• density_matrix=True simulations
• noisy simulation and collapsing measurements

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (45e4e22) 99.96% compared to head (fcf4747) 99.90%.

Files	Patch %	Lines
src/qibo/backends/einsum_utils.py	76.92%	3 Missing ⚠️
src/qibo/backends/numpy.py	94.73%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1186      +/-   ##
==========================================
- Coverage   99.96%   99.90%   -0.06%     
==========================================
  Files          70       70              
  Lines       10414    10438      +24     
==========================================
+ Hits        10410    10428      +18     
- Misses          4       10       +6     

Flag	Coverage Δ
unittests	99.90% <93.15%> (-0.06%)	⬇️

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