Add exp to engine.Value

[ritesh99rakesh]

I have added exp function using math.exp in micrograd/engine.py

I will add tests once the changes are approved.

@karpathy, please review

[wangcongcong123]

There is pow method. Why still needs this method? Just curious.

[karpathy]

because pow currently requires the second argument to be an int/float.

[dhern023]

I will add tests once the changes are approved.

No one would /should approve untested code.