Registers a flag whose value must be a float.

If lower_bound or upper_bound are set, then this flag must be within the given range.

name str, the flag name.
default float|str|None, the default value of the flag.
help str, the help message.
lower_bound float, min value of the flag.
upper_bound float, max value of the flag.
flag_values FlagValues, the FlagValues instance with which the flag will be registered. This should almost never need to be overridden.
**args dict, the extra keyword args that are passed to DEFINE.

a handle to defined flag.