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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _warmup(self, num_updates):
 """ Returns linearly increasing fraction of base_lr. """

        assert self.base_lr is not None
        if not self.warmup:
            return self.base_lr
        fraction = (num_updates + 1) * self.base_lr / (self.warmup + 1)
        if num_updates > self.last_warmup_log and num_updates % self.log_warmup_every_t == 0:
            self.last_warmup_log = num_updates
            logger.info("Learning rate warmup: %3.0f%%", fraction / self.base_lr * 100.0)
        return fraction