The function mapping_fn is applied separately across the group of devices
represented by the placement type of value. For example, if value has
placement type tff.CLIENTS, then mapping_fn is applied to each client
individually. In particular, this operation does not alter the placement of
the federated value.
A mapping function to apply pointwise to member constituents of
value. The parameter of this function must be of the same type as the
member constituents of value.
A value of a TFF federated type (or a value that can be implicitly
converted into a TFF federated type, e.g., by zipping) placed at
tff.CLIENTS or tff.SERVER.
A federated value with the same placement as value that represents the
result of mapping_fn on the member constituent of arg.
If the arguments are not of the appropriate types.