Annotate activation functions with Nx blocks

[seanmor5]

This is related to the introduction of Nx.block and custom calls. To make this transition easier, I added a defblock construct which marks a function as an Nx.block with a generated camel-case module name equivalent to the function name.

The plan would be to use this for all layers as well, but that requires some changes because some layers are deftransform instead of defn.

Axon blocks will be prefixed with Axon.Block. It's confusing a bit because these are different than Axon.block

[polvalente]

@josevalim @seanmor5 wdyt about having a single Axon.Block struct with module, function and arity as the name?

%Axon.Block{module: Axon.Activations, function: :softplus, arity: 1, options: ...}

%Axon.Block{module: Axon.Layers, function: :dense, arity: 2, options: ...}

or something like that