Metrics general supplementary checking before `compute`

As discussed here, the following assumption:

All metrics that has not been updated yet after last reset should raise NotComputableError when compute is called

holds for any metric.
Today we check metric specific variable _num_examples if it's equal zero and raise exception if it is the case. This behaviour can be replaced directly in Metric by counting the number of update calls between reset and before compute.
An example implementation is proposed by @zasdfgbnm :

class Metric:
    def  __init__(self, ...):
        ......

        def wrapped_reset():
            self._updated = False
            self.reset()
        self.reset = functools.wraps(self.reset)(wrapped_reset)

        def wrapped_update(output):
            self._updated = True
            self.update(output)
        self.update =  functools.wraps(self.update)(wrapped_update)

        def wrapped_compute():
            if not self._updated:
                raise NotComputableError('not updated before compute')
            return self.compute()
        self.compute =  functools.wraps(self.compute)(wrapped_compute)

Let's discuss here what we can do.

cc @zasdfgbnm

Start by reading the Metric implementation and the metric-specific _num_examples checks described in the issue, then review the linked pull request discussion for the proposed wrapping approach. Done means the pre-compute update check is handled centrally in Metric and the existing NotComputableError behavior is preserved for metrics that were not updated.