Inadequate total steps shown in progress bar during sanity validation check when number of validation dataloaders >= 2
Describe the bug
The number of total steps shown in progress bar during validation sanity check is underestimated when multiple validation dataloaders are provided.
To Reproduce
Steps to reproduce the behavior:

At def val_dataloader(self), return a list of >= 2 validation dataloaders.
e.g. return [val_dataloader1, val_dataloader2]
Set nb_sanity_val_steps for trainer to n  (default n = 5)
Run experiment

Expected behavior

If two validation dataloaders are provided, the total steps shown in progress bar during validation sanity check are n * 2. That is, 10 steps shown progress bar.

In current master - trainer.py
if self.get_val_dataloaders() is not None and self.num_sanity_val_steps > 0:
            # init progress bars for validation sanity check
            pbar = tqdm.tqdm(desc='Validation sanity check', total=self.num_sanity_val_steps,
                             leave=False, position=2 * self.process_position,
                             disable=not self.show_progress_bar, dynamic_ncols=True, unit='batch')

Should be modified to
if self.get_val_dataloaders() is not None and self.num_sanity_val_steps > 0:
            # init progress bars for validation sanity check
            pbar = tqdm.tqdm(desc='Validation sanity check', total=self.num_sanity_val_steps*len(self.get_val_dataloaders()),
                             leave=False, position=2 * self.process_position,
                             disable=not self.show_progress_bar, dynamic_ncols=True, unit='batch')