cudnn.Tanh about cudnn.torch

[soumith]

@ibmua Tanh usually takes close to negligible time compared to the rest of the network, so it doesn't really matter practically.

[ibmua]

But if it's actually slower in the cudnn implementation, then I guess it's a good idea to remove it from the documentation and to remove other slower equivalents of standart functions if such happen to exist. So as to not make code slower while making it simultaneosly more obfuscated. And, probably, to change cudnn.Tanh to be equivalent to nn.Tanh - not delete it, but still make it equal to the faster version - for code legacy reasons.