Change .ravel() back to .flatten() for gates

[stavros11]

The latest version of qibojit fails when running multigpu circuits on a machine with multiple physical GPUs. For example the following minimal example:

from qibo import models, gates

circuit = models.Circuit(3, accelerators={"/GPU:0": 1, "/GPU:1": 1})
circuit.add(gates.H(0))

final_state = c()
print(final_state)

fails with

cupy_backends.cuda.api.runtime.CUDARuntimeError: cudaErrorIllegalAddress: an illegal memory access was encountered

After checking earlier qibojit versions I realized that the error is associated with #74 and particularly because .flatten() was changed to .ravel() for some gate castings. This is fixed here and the above example works. Also, all tests are passing.

@scarrazza @andrea-pasquale, if you have access to a machine with multiple physical GPUs it would be good if you could check if the latest qibojit main gives you the above error and confirm that this PR fixes it. I found this while running the qibo tests on DGX. Note that the machine needs to have at least two GPUs, if there is only one (or the second GPU is hidden with CUDA_VISIBLE_DEVICES) then the error will not appear.

[codecov]

Codecov Report

Merging #79 (f7afbec) into main (d3ff74e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #79   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines         1088      1088           
=========================================
  Hits          1088      1088           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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Impacted Files	Coverage Δ
src/qibojit/custom_operators/platforms.py	100.00% <ø> (ø)

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[andrea-pasquale]

Thanks for spotting this @stavros11.
I can confirm that main fails on dom when running with 2 GPUs.
This PR fixes the issue.