Dynamic dispatching in optimizers

I could be wrong in this, but looking at the code for [sgd](https://github.com/dmlc/MXNet.jl/blob/master/src/optimizers/sgd.jl) and [adam](https://github.com/dmlc/MXNet.jl/blob/master/src/optimizers/adam.jl) it looks like there is dynamic function dispatch going on because the types for optimizer options are not known at compile time.

For example for sgd in `update`:

``` Julia
lr = get_learning_rate(self.opts.lr_scheduler, self.state)
```

If I understand Julia correctly, then `lr_scheduler` is a boxed value in an `Any` block and thus the appropriate `get_learning_rate` variant needs to be looked up at runtime, which can have performance implications.

What do you think?