Installation instructions for cuQuantum backend

[andrea-pasquale]

This PR is related to the cuQuantum backend from qiboteam/qibojit#57.
I've updated the installation instructions and I've added cuQuantum along the other platforms available in qibojit.
Let me know what you think.

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[mlazzarin]

Thanks! Looks good to me, only one comment.

[mlazzarin]

Shall we clarify that our cuquantum backend is an alternative GPU implementation, so that installing it is not required to have GPU acceleration (in contrast to cupy)?

[stavros11]

I agree with Marco's comment, we can add a sentence where we say that cuquantum is optional. Other than that it looks good, thanks.

In #533 I document how one can switch between cupy and cuquantum using qibo.set_backend(). At some point we should also document the default switch using environment variables, along with other environment variables we have in qibo. I'm not sure if this should be done here or in a different PR.

[andrea-pasquale]

Thank you both for the feedback!
As @stavros11 suggested, I've added a sentence both in the pip and conda note saying that cuQuantum is an optional dependency. Let me know if there is something else to change.

In #533 I document how one can switch between cupy and cuquantum using qibo.set_backend(). At some point we should also document the default switch using environment variables, along with other environment variables we have in qibo. I'm not sure if this should be done here or in a different PR.

If you want to add it directly to #533 I am fine with it. Otherwise I can add everything related to the documentation directly here.

[scarrazza]

@andrea-pasquale thanks, if you include the and/or this might increase readability.

[andrea-pasquale]

@andrea-pasquale thanks, if you include the and/or this might increase readability.

Thanks for the feedback. I've added your suggestions.