Add tf.LeakyRelu.

* Add tf.LeakyRelu op definition + folders (well one is really canonicalizer)
* Change generated error message to use attribute description instead;
* Change the return type of F32Attr to be APFloat - internally it is already
  stored as APFloat so let the caller decides if they want to convert it or
  not. I could see varying opinions here though :) (did not change i32attr
  similarly)

PiperOrigin-RevId: 232923358