Issue title: Missing description of parameter
 Issue body: The parameter `target_label` here is missing some description:

https://deeprobust.readthedocs.io/en/latest/source/deeprobust.image.attack.html#deeprobust.image.attack.lbfgs.LBFGS.generate

I thought it would accept an array of the same shape as `label`, it turns out to accept only an integer.
 Comments: 
 Comment 0: Also, this description in the CW class looks off:

        label :
            target label

https://deeprobust.readthedocs.io/en/latest/source/deeprobust.image.attack.html#deeprobust.image.attack.cw.CarliniWagner.generate
 Comment 1: First, thank you for reminding us for the missing description! 
Currently LBFGS algorithm does not support batch calculation. The reason is that for each generating process, it is required to search for the result of the box-constrained optimization problem. 
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