Force Float32 as type presented to Flux chains

[ablaom]

This PR replaces #272 and the fix #274, resolving #275.

[ablaom]

Thanks @EssamWisam, much appreciated.

I will merge this, but just note there is some noise in the test log because of some method redefinitions in test/entity_embedding.jl. They occur because of that loop you have over finalizer = [softmax, relu]. It's only bothering me because I'm likely to forget later what the source of the complaint is and waste time re-investigating. Be great if you could clean this up some time.