Normalize supports arbitrary dimensions

[fmassa]

Adds docs + tests.
Also, cleans a bit some unnecessary temporary variables.

Important remark: I added the dimension parameter as the second argument to the constructor, making the eps the third one. This breaks code that changed the default eps during initialization, but I felt that dim coming after eps felt weird. Let me know if you prefer to let dim the last argument.

[apaszke]

@fmassa I think if you're changing the order of arguments like here, it's worth adding a check that dim is an integer. This would at least throw an error if somebody used the eps argument now.

[fmassa]

@apaszke I agree. just pushed a diff checking that dim is integer.

[fmassa]

I forgot to consider that previously saved models do not have the dimfield. I'm pushing a patch

[fmassa]

Fixed case of old saved models. To make it simple I just added the default value in the forward/backward calls, I didn't overwrite the read method.

[fmassa]

One note about the negative indexes, if you want I could also add a numInputDims parameter, as in nn.Max. Let me know what you think.