Writes an argument value into an FloatArg proto.

Note that the FloatArg proto is a slimmed down form of the Arg proto, so this proto should only be used when the argument is known to be a float or expression that resolves to a float.

value The value to encode. This must be a float or compatible sympy expression. Strings and repeated booleans are not allowed.
arg_function_language The language to use when encoding functions. If this is set to None, it will be set to the minimal language necessary to support the features that were actually used.
out The proto to write the result into. Defaults to a new instance.

The proto that was written into.