This callback method must be implemented by all subclasses. This
is the method that will be invoked on every Tuple of a given dataset.
Since the dataset may be divided up in a variety of ways the programmer
should not make assumptions about state that is maintained between
invocations of this method.
Whether the UDF should be evaluated at compile time if all inputs are constant.
This is applicable for most UDF, however, if a UDF will access hdfs file which
is not available at compile time, it has to be false