Fast gradient Method doesn't work for double precision: Suggestion for the fix

[williampiat3]

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Describe the bug
I used the PGD method in this folder but I use double precision in my models thus encountered an error while using the attack
cleverhans/cleverhans/torch/attacks/projected_gradient_descent.py

I found a way of fixing the problem:
line 74 in cleverhans/cleverhans/torch/attacks/fast_gradient_method.py
instead of:

x = x.clone().detach().to(torch.float).requires_grad_(True)

I put

x = x.clone().detach().to(x.dtype).requires_grad_(True)

My solution then covers double precision and simple precision

[kylematoba]

this also breaks for torch.float16.